py/objstr: Optimize string concatenation with empty string.

In this, don't allocate copy, just return non-empty string. This helps
with a standard pattern of buffering data in case of short reads:

    buf = b""
    while ...:
        s = f.read(...)
        buf += s
        ...

For a typical case when single read returns all data needed, there won't
be extra allocation. This optimization helps uasyncio.