patch 8.2.1431: Vim9: no error for white space before comma in dict

Problem:    Vim9: no error for white space before comma in dict.
Solution:   Check for extra white space. (closes #6674)
diff --git a/src/dict.c b/src/dict.c
index b6c9016..a5569e6 100644
--- a/src/dict.c
+++ b/src/dict.c
@@ -922,7 +922,12 @@
 	if (!had_comma)
 	{
 	    if (evaluate)
-		semsg(_(e_missing_dict_comma), *arg);
+	    {
+		if (**arg == ',')
+		    semsg(_(e_no_white_before), ",");
+		else
+		    semsg(_(e_missing_dict_comma), *arg);
+	    }
 	    goto failret;
 	}
     }