patch 8.2.4672: using :normal with Ex mode may make :substitute hang

Problem:    Using :normal with Ex mode may make :substitute hang.
Solution:   When getting an empty line behave like 'q' was typed.
            (closes #10070)

src/ex_cmds.c

@@ -4233,6 +4233,11 @@ ex_substitute(exarg_T *eap)
 			    {
 				typed = *resp;
 				vim_free(resp);
+				// When ":normal" runs out of characters we get
+				// an empty line.  Use "q" to get out of the
+				// loop.
+				if (ex_normal_busy && typed == NUL)
+				    typed = 'q';
 			    }
 			}
 			else