Instead of doing return, insert an if block to skip emitting zero fields.

This also allows the check to be inserted at any level.

Fixes tinylib#376

Emitted code:

```
// MarshalMsg implements msgp.Marshaler
func (z TypeSample) MarshalMsg(b []byte) (o []byte, err error) {
	o = msgp.Require(b, z.Msgsize())
	// check for omitted fields
	zb0001Len := uint32(2)
	var zb0001Mask uint8 /* 2 bits */
	_ = zb0001Mask
	if z.K == 0 {
		zb0001Len--
		zb0001Mask |= 0x1
	}
	if z.V == 0 {
		zb0001Len--
		zb0001Mask |= 0x2
	}
	// variable map header, size zb0001Len
	o = append(o, 0x80|uint8(zb0001Len))
	// Skip if no fields are to be emitted
	if zb0001Len != 0 {
		if (zb0001Mask & 0x1) == 0 { // if not omitted
			// string "k"
			o = append(o, 0xa1, 0x6b)
			o = msgp.AppendUint32(o, z.K)
		}
		if (zb0001Mask & 0x2) == 0 { // if not omitted
			// string "v"
			o = append(o, 0xa1, 0x76)
			o = msgp.AppendUint32(o, z.V)
		}
	}
	return
}
```

If only 1 field, the check is omitted (and there is similar behavior on clearomitted).