When output is empty, exec should return "", not "\0" (#5775)

@- Fixed exec() returning "\0" when output is empty

ext/standard/exec.c

@@ -172,8 +172,9 @@ static int _Exec(int type, char *cmd, pval *array, pval *return_value)
 
 			tmp = php_addslashes(buf, 0, &len, 0);
 			RETVAL_STRINGL(tmp,len,0);
-		} else
-			RETVAL_STRINGL(buf,l+1,1);
+		} else {
+			RETVAL_STRINGL(buf,l?l+1:0,1);
+		}
 	} else {
 		int b, i;