Use 'is not None' guards so empty-string sources still validate

Per copilot review on PR DOI-USGS#246. The previous truthiness guard skipped
validation for empty-string feature_source/data_source, letting bad
inputs build invalid URLs.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: thodson-usgs

dataretrieval/nldi.py

@@ -256,10 +256,10 @@ def get_features(
         # validate the feature source and comid
         _validate_feature_source_comid(feature_source, feature_id, comid)
         # validate the data source
-        if data_source:
+        if data_source is not None:
             _validate_data_source(data_source)
         # validate feature source
-        if feature_source:
+        if feature_source is not None:
             _validate_data_source(feature_source)
         # validate the navigation mode
         if navigation_mode: