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
```

tests/jq.test

@@ -1036,6 +1036,19 @@ null
 null
 [5,6]
 
+[if true then 3 end]
+7
+[3]
+
+[if false then 3 end]
+7
+[7]
+
+[if false then 3 else . end]
+7
+[7]
+
+
 [.[] | [.foo[] // .bar]]
 [{"foo":[1,2], "bar": 42}, {"foo":[1], "bar": null}, {"foo":[null,false,3], "bar": 18}, {"foo":[], "bar":42}, {"foo": [null,false,null], "bar": 41}]
 [[1,2], [1], [3], [42], [41]]