Allow empty libraries sections.

Data_submission/build_file_dicts.py

@@ -240,6 +240,8 @@ def main():
             # determine raw data files for this analysis
             ###
             raws_wanted = set()
+            # If empty, not read as a dict and doesn't have items
+            if analysis_info["libraries"]:
                 # library: sample name for submission
                 for (library, rawdat) in analysis_info["libraries"].items():
                     assert Path(rawdat).exists()