doc: check if the keyword is non-empty

Exit with error if a parsed keyword is an empty string.
Based on grass-dev: man error on compilation
https://lists.osgeo.org/pipermail/grass-dev/2016-November/083153.html


git-svn-id: https://svn.osgeo.org/grass/grass/trunk@69871 15284696-431f-4ddb-bdfa-cd5b030d7da7

man/build_keywords.py

@@ -42,6 +42,9 @@ for fname in htmlfiles:
             key = key.split('>')[1].split('<')[0]
         except:
             pass
+        if not key:
+            exit("Empty keyword from file %s line: %s"
+                 % (fname, lines[index_keys]))
         if key not in keywords.keys():
             keywords[key] = []
             keywords[key].append(fname)