fix: use is not None guard for empty frontmatter dict validation

`frontmatter={}` is a valid input where required keys should be flagged
missing. The truthiness check `if frontmatter` skipped validation for
empty dicts, allowing strict-mode to incorrectly pass.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: phernandez <paul@basicmachines.co>

Author: phernandez

src/basic_memory/schema/validator.py

@@ -119,7 +119,7 @@ def validate_note(
     # Trigger: schema has frontmatter_fields and caller provided frontmatter dict
     # Why: settings.frontmatter rules validate metadata keys like tags, status
     # Outcome: frontmatter fields produce the same FieldResult/warning/error as content fields
-    if frontmatter and schema.frontmatter_fields:
+    if frontmatter is not None and schema.frontmatter_fields:
         for fm_field in schema.frontmatter_fields:
             field_result = _validate_frontmatter_field(fm_field, frontmatter)
             result.field_results.append(field_result)

tests/schema/test_validator.py

@@ -360,6 +360,23 @@ def test_frontmatter_none_skips_validation(self):
         assert result.passed is True
         assert result.field_results == []
 
+    def test_empty_frontmatter_dict_validates_missing_keys(self):
+        """frontmatter={} is a valid input — required keys should be flagged missing."""
+        fm_field = SchemaField(name="status", type="string", required=True)
+        schema = self._make_fm_schema([fm_field])
+        result = validate_note("test-note", schema, [], [], frontmatter={})
+        assert result.field_results[0].status == "missing"
+        assert len(result.warnings) == 1
+        assert "status" in result.warnings[0]
+
+    def test_empty_frontmatter_dict_strict_fails(self):
+        """frontmatter={} in strict mode should produce errors for required keys."""
+        fm_field = SchemaField(name="status", type="string", required=True)
+        schema = self._make_fm_schema([fm_field], validation_mode="strict")
+        result = validate_note("test-note", schema, [], [], frontmatter={})
+        assert result.passed is False
+        assert len(result.errors) == 1
+
     def test_empty_frontmatter_fields_skips_validation(self):
         schema = self._make_fm_schema([])
         result = validate_note("test-note", schema, [], [], frontmatter={"status": "draft"})