Fix static-analyzer error: +[DDLog registeredClasses] returning nil

+[DDLog registeredClasses] should return empty array rather than nil.
Is declared between NS_ASSUME_NONNULL_BEGIN and NS_ASSUME_NONNULL_END

Classes/DDLog.m

@@ -609,7 +609,7 @@ + (NSArray *)registeredClasses {
 
         classes = numClasses ? (Class *)malloc(sizeof(Class) * bufferSize) : NULL;
         if (classes == NULL) {
-            return nil; //no memory or classes?
+            return @[]; //no memory or classes?
         }
 
         numClasses = (NSUInteger)MAX(objc_getClassList(classes, (int)bufferSize),0);