csv_metadata_quality/app.py: Don't crash if fields are missing

We don't need to crash if someone feeds us a CSV file that is miss-
ing commont DSpace fields like title, type, and subject.

csv_metadata_quality/app.py

@@ -164,6 +164,7 @@ def run(argv):
 
     # Check: duplicate items
     # We extract just the title, type, and date issued columns to analyze
+    try:
         duplicates_df = df.filter(
             regex=r"dcterms\.title|dc\.title|dcterms\.type|dc\.type|dcterms\.issued|dc\.date\.issued"
         )
@@ -171,6 +172,8 @@ def run(argv):
 
         # Delete the temporary duplicates DataFrame
         del duplicates_df
+    except IndexError:
+        pass
 
     ##
     # Perform some checks on rows so we can consider items as a whole rather