fix(02-03): treat null foreign keys as valid masked ids

PHP album payloads leave discogs_id null. The id mask required an
integer and failed PHP self-replay on otherwise identical bodies.

Co-authored-by: Cursor <cursoragent@cursor.com>

tide/normalize.go

@@ -81,6 +81,9 @@ func maskDate(path string, val any, diffs *[]Diff) any {
 }
 
 func maskIDValue(path string, val any, diffs *[]Diff) any {
+	if val == nil {
+		return nil
+	}
 	n, ok := val.(json.Number)
 	if !ok {
 		*diffs = append(*diffs, Diff{Path: path, Expected: "integer id", Actual: formatValue(val)})