warning needed when nothing passes the filter

[g1o]

After make_correlation_plot.R, table.Rmd will fail if there is nothing.

idx <- which(
# q-value cutoffs
(plotframe$corrected_contrasts < defs$linear_model.qvalue.cutoff) &
(plotframe$Spearman_qvalue < defs$spearman.qvalue.cutoff) &
(plotframe$Pearson_qvalue < defs$pearson.qvalue.cutoff) &
(plotframe$Kendall_qvalue < defs$kendall.qvalue.cutoff) &
# correlation cutoffs
((plotframe$Pearson_cor > defs$pearson.cor.upper.cutoff) | (plotframe$Pearson_cor < defs$pearson.cor.lower.cutoff)) &
((plotframe$Spearman_cor > defs$spearman.cor.upper.cutoff) | (plotframe$Spearman_cor < defs$spearman.cor.lower.cutoff)) &
((plotframe$Kendall_cor > defs$kendall.cor.upper.cutoff) | (plotframe$Kendall_cor < defs$kendall.cor.lower.cutoff)) &
# basic statistics cutoffs
(plotframe$size > defs$annotation_size.cutoff) &
(plotframe$prevalence > defs$prevalence.cutoff) &
(plotframe$heterogeneity > defs$heterogeneity.cutoff) &
(plotframe$sd > defs$sd.cutoff) &
(plotframe$cv > defs$cv.cutoff))