patch 8.2.0517: Vim9: cannot separate "func" and "func(): void"

Problem:    Vim9: cannot separate "func" and "func(): void".
Solution:   Use VAR_ANY for "any" and VAR_UNKNOWN for "no type".

src/json.c

@@ -351,6 +351,7 @@ json_encode_item(garray_T *gap, typval_T *val, int copyID, int options)
 	    break;
 #endif
 	case VAR_UNKNOWN:
+	case VAR_ANY:
 	case VAR_VOID:
 	    internal_error_no_abort("json_encode_item()");
 	    return FAIL;