Accomodate for possible absence of small RNA type.

small_RNA-seq/small_RNA-seq.snakefile

@@ -2059,7 +2059,9 @@ def source_small_RNA_counts(wildcards):
 def make_tag_association(dfs, tag):
     """Associates a tag "tag" to the union of the indices of dataframes *dfs*."""
     idx = reduce(union, (df.index for df in dfs))
-    return pd.DataFrame(list(zip(idx, repeat(tag)))).set_index(0)
+    if len(idx):
+        return pd.DataFrame(list(zip(idx, repeat(tag)))).set_index(0)
+    return pd.DataFrame()
 
 
 # Could we use biotype instead?