fix: Add attribute nonnull_error if returning pointer and error

Signed-off-by: jefft0 <[email protected]>

Author: jefft0

bind/genobjc.go

@@ -582,6 +582,11 @@ func (s *funcSummary) asSignature(g *ObjcGen) string {
 		}
 		params = append(params, fmt.Sprintf("%s:(%s)%s", key, g.objcType(p.typ)+"* _Nullable", p.name))
 	}
+	if len(params) > 0 && params[len(params)-1] == ":(NSError* _Nullable* _Nullable)error" &&
+		(s.ret == "NSData* _Nullable" || s.ret == "NSString* _Nullable") {
+		// Allow returning nil to Swift.
+		params = append(params, "__attribute__((swift_error(nonnull_error)))")
+	}
 	return strings.Join(params, " ")
 }