Guard onFieldSubmitted on non-empty text to prevent cleared values reappearing

When a user clears a tag value and presses Done, RawAutocomplete's
onFieldSubmitted auto-selects the first option from the suggestions
list. Since optionsBuilder returns all suggestions for empty text,
this causes the cleared value to reappear. Guarding the call on
non-empty text prevents the auto-selection while preserving
autocomplete behavior when the user has typed a partial match.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/widgets/nsi_tag_value_field.dart

@@ -132,7 +132,14 @@ class _NSITagValueFieldState extends State<NSITagValueField> {
           onChanged: (value) {
             widget.onChanged(value);
           },
-          onSubmitted: (_) => onFieldSubmitted(),
+          onSubmitted: (_) {
+            // Only auto-complete when there's text to match against.
+            // Otherwise, pressing Done on an empty field would auto-select
+            // the first suggestion, preventing users from clearing values.
+            if (controller.text.isNotEmpty) {
+              onFieldSubmitted();
+            }
+          },
         );
       },
       optionsViewBuilder: (