fix(subst/variable): invalid identifiers were having their $ swallowed

It is safe to delete this part:
```c
if (!rep)
   return (NULL);
```
because rep is always returned, and no other code depends on that value

test.sh

@@ -658,4 +658,20 @@ expecting <<EOF
 hi
 EOF
 
+when_run <<EOF "variable substitution with no valid name is not substitued"
+echo \$=
+echo \$:
+EOF
+expecting <<EOF
+$=
+$:
+EOF
+
+when_run <<EOF "variable substitution with a lone dollar sign"
+echo " $ " | cat -e
+EOF
+expecting <<EOF
+ $ $
+EOF
+
 finalize