fix: revert null-aware map entries; disable use_null_aware_elements lint

The ? operator on map entries applies to the key, not the value.
String literal keys can never be null, so ?'key': value was flagged as
invalid_null_aware_operator. Reverted to if (x != null) 'key': x form
and disabled the lint project-wide since it doesn't apply here.

analysis_options.yaml

@@ -23,6 +23,9 @@ linter:
   rules:
     # avoid_print: false  # Uncomment to disable the `avoid_print` rule
     # prefer_single_quotes: true  # Uncomment to enable the `prefer_single_quotes` rule
+    # ?'key': value applies ? to the key (never null for string literals);
+    # the if (x != null) form is correct for nullable-value map entries.
+    use_null_aware_elements: false
 
 # Additional information about this file can be found at
 # https://dart.dev/guides/language/analysis-options