patch 8.2.0103: using null object with execute() has strange effects

Problem:    Using null object with execute() has strange effects.
Solution:   Give an error message ofr Job and Channel.
diff --git a/src/eval.c b/src/eval.c
index 6e88c5c..e0f27a0 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -5667,7 +5667,7 @@
 #endif
 	    break;
 	case VAR_UNKNOWN:
-	    emsg(_("E908: using an invalid value as a String"));
+	    emsg(_(e_inval_string));
 	    break;
     }
     return NULL;