Remove labels on NULL plots using plot_grid and cowplot

Here is a potential solution using a modified cowplot::plot_grid():

plot_grid_modified <- function(..., plotlist = NULL, align = c("none", "h", "v", "hv"),
                      axis = c("none", "l", "r", "t", "b", "lr", "tb", "tblr"),
                      nrow = NULL, ncol = NULL, rel_widths = 1,
                      rel_heights = 1, labels = NULL, label_size = 14,
                      label_fontfamily = NULL, label_fontface = "bold", label_colour = NULL,
                      label_x = 0, label_y = 1,
                      hjust = -0.5, vjust = 1.5, scale = 1., greedy = TRUE,
                      byrow = TRUE, cols = NULL, rows = NULL) {
  
  # Make a list from the ... arguments and plotlist
  plots <- c(list(...), plotlist)
  num_plots <- length(plots)
  
  if (!is.null(cols)){
    warning("Argument 'cols' is deprecated. Use 'ncol' instead.")
  }
  
  if (!is.null(rows)){
    warning("Argument 'rows' is deprecated. Use 'nrow' instead.")
  }
  
  scale <- rep_len(scale, num_plots)
  if (sum(scale <= 0) > 1){
    stop("Argument 'scale' needs to be greater than 0.")
  }
  
  # internally, this function operates with variables cols and rows instead of ncol and nrow
  if (!is.null(ncol)){
    cols <- ncol
  }
  if (!is.null(nrow)){
    rows <- nrow
  }
  
  
  # calculate grid dimensions
  if (is.null(cols) && is.null(rows)){
    # if neither rows nor cols are given, we make a square grid
    cols <- ceiling(sqrt(num_plots))
    rows <- ceiling(num_plots/cols)
  }
  # alternatively, we know at least how many rows or how many columns we need
  if (is.null(cols)) cols <- ceiling(num_plots/rows)
  if (is.null(rows)) rows <- ceiling(num_plots/cols)
  
  # if the user wants to layout the plots by column, we use the calculated rows to reorder plots
  if (!isTRUE(byrow)) plots <- plots[c(t(matrix(c(1:num_plots, rep(NA, (rows * cols) - num_plots)), nrow = rows, byrow = FALSE)))]
  
  # Align the plots (if specified)
  grobs <- align_plots(plotlist = plots, align = align, axis = axis, greedy = greedy)
  
  if ("AUTO" %in% labels) {
    count <- 0
    labels <- c()
    for (idx in seq_along(plots)) {
      if (!is.null(unlist(plots[idx]))) {
        count <- count + 1
        labels <- c(labels, LETTERS[count])
      } else {
        labels <- c(labels, "")
      }
    }
  } else if ("auto" %in% labels) {
    count <- 0
    labels <- c()
    for (idx in seq_along(plots)) {
      if (!is.null(unlist(plots[idx]))) {
        count <- count + 1
        labels <- c(labels, letters[count])
      } else {
        labels <- c(labels, "")
      }
    }
  }

  # label adjustments can be provided globally for all labels
  # or individually for each label
  hjust <- rep_len(hjust, length(labels))
  vjust <- rep_len(vjust, length(labels))
  label_x <- rep_len(label_x, length(labels))
  label_y <- rep_len(label_y, length(labels))
  
  # calculate appropriate vectors of rel. heights and widths
  rel_heights <- rep(rel_heights, length.out = rows)
  rel_widths <- rep(rel_widths, length.out = cols)
  # calculate the appropriate coordinates and deltas for each row and column
  x_deltas <- rel_widths/sum(rel_widths)
  y_deltas <- rel_heights/sum(rel_heights)
  xs <- cumsum(rel_widths)/sum(rel_widths) - x_deltas
  ys <- 1 - cumsum(rel_heights)/sum(rel_heights)
  
  # now place all the plots
  p <- ggdraw() # start with nothing
  col_count <- 0
  row_count <- 1
  for (i in 1:(rows*cols)){
    if (i > num_plots) break
    
    x_delta <- x_deltas[col_count+1]
    y_delta <- y_deltas[row_count]
    x <- xs[col_count+1]
    y <- ys[row_count]
    
    # place the plot
    p_next <- grobs[[i]]
    if (!is.null(p_next)){
      p <- p + draw_grob(p_next, x, y, x_delta, y_delta, scale[i])
    }
    # place a label if we have one
    if (i <= length(labels)){
      p <- p + draw_plot_label(labels[i], x + label_x[i]*x_delta, y + label_y[i]*y_delta, size = label_size,
                               family = label_fontfamily, fontface = label_fontface, colour = label_colour,
                               hjust = hjust[i], vjust = vjust[i])
    }
    # move on to next grid position
    col_count <- col_count + 1
    if (col_count >= cols){
      col_count <- 0
      row_count <- row_count + 1
    }
  }
  p
}

library(ggplot2)
library(cowplot)

df <- data.frame(
  x = 1:10, y1 = 1:10, y2 = (1:10)^2, y3 = (1:10)^3, y4 = (1:10)^4
)

p1 <- ggplot(df, aes(x, y1)) + geom_point()
p2 <- ggplot(df, aes(x, y2)) + geom_point()
p3 <- ggplot(df, aes(x, y3)) + geom_point()
p4 <- ggplot(df, aes(x, y4)) + geom_point()
p5 <- ggplot(mpg, aes(as.factor(year), hwy)) +
  geom_boxplot() +
  facet_wrap(~class, scales = "free_y")
# simple grid
plot_grid_modified(p1, NULL, p3, p4, labels = "AUTO")