Fixes empty string bug in str isalpha method

[advikkabra]

In the current implementation of the isalpha() method, an empty string returns a True value, but in CPython it returns False.

[anutosh491]

Good catch @advikkabra
Do you see any more builtin methods where we might be going wrong in corner cases like this ?
Would be great if you could give it a look.

Also the CI failing for the macos build isn't due to your commit, so no need to worry about it.