[analyzer] Fix parse exception when file doesn't exist
If a source file referenced by a plist does not exist, the CodeChecker parse
was thrown the following exception:
```
TypeError: '>' not supported between instances of 'NoneType' and 'float'
```
This commit solves this problems and check whether the variable is None.
@@ -523,6 +523,11 @@ def parse(plist_file, metadata_dict, rh, file_report_map):
continue
file_mtime = util.get_last_mod_time(source_file)
+ if not file_mtime:
+ changed_files.add(source_file)
+ LOG.warning('%s does not exist.', source_file)
+ continue
+
if file_mtime > plist_mtime:
changed_files.add(source_file)
LOG.warning('%s did change since the last analysis.', source_file)
