Support "if" without "else" clause

Defaults the else clause to the identity filter "." if an else clause is
missing.

The following two jq programs are considered equivalent:

```
if .foo then
    .foo = "bar"
else
    .
end
```

```
if .foo then
    .foo = "bar"
end
```

src/parser.y

@@ -357,6 +357,9 @@ Term "as" Patterns '|' Exp {
 "if" Exp "then" Exp ElseBody {
   $$ = gen_cond($2, $4, $5);
 } |
+"if" Exp "then" Exp "end" {
+  $$ = gen_cond($2, $4, gen_noop());
+} |
 "if" Exp "then" error {
   FAIL(@$, "Possibly unterminated 'if' statement");
   $$ = $2;