Merge pull request #512 from jedwards4b/user_mods_path_fix

This fixes a bug when the user_mods_path is unset in a case or a test

Test suite: scripts_regression_tests
Test baseline:
Test namelist changes:
Test status: bit for bit

Fixes #509

User interface changes?:

Code review: jayeshkrishna

utils/python/CIME/user_mod_support.py

@@ -92,12 +92,14 @@ def build_include_dirs_list(user_mods_path, include_dirs=None):
     each of those directories for further directories.
     The file may also include comments deleneated with # in the first column
     '''
+    include_dirs = [] if include_dirs is None else include_dirs
+    if user_mods_path is None or user_mods_path == 'UNSET':
+        return include_dirs
     expect(os.path.isabs(user_mods_path),
            "Expected full directory path, got '%s'"%user_mods_path)
     expect(os.path.isdir(user_mods_path),
            "Directory not found %s"%user_mods_path)
     logger.info("Adding user mods directory %s"%user_mods_path)
-    include_dirs = [] if include_dirs is None else include_dirs
     include_dirs.append(os.path.normpath(user_mods_path))
     include_file = os.path.join(include_dirs[-1],"include_user_mods")
     if os.path.isfile(include_file):