Fix bug if reads jump over gene

If they a given read doesn't support any exons, than it shouldn't
support excluding them either. Imagine a case like

Gene A: >>>>>------------->>>>
Gene B:       <<<---<<<

A read from gene A shouldn't tell you anything about the splicing of
gene B.

Author: petercombs

CalculatePSI.py

@@ -55,7 +55,11 @@ def get_supported_exons(read, exons_in_gene):
         #print("StoppedIteration", cur_exon)
         pass
     finally:
-        return supported_lens, supported_exons, excluded_exons
+        return (
+            supported_lens,
+            supported_exons,
+            (excluded_exons if supported_exons else [])
+        )
 
 
 def get_exon_dictionary(gtf_file):