Fix adding empty Constant node after FormattedValue

[lysnikolaou]

Before this PR, there was an empty Constant node after the FormattedValue node even for a simple f-string like f'{a}':

>>> f = 'f"{a}"'
>>> ast.dump(ast.parse(f))
"Module(body=[Expr(value=JoinedStr(values=[FormattedValue(value=Name(id='a', ctx=Load()), conversion=-1), Constant(value='')]))], type_ignores=[])"

With this PR:

>>> f = 'f"{a}"'
>>> ast.dump(ast.parse(f))
"Module(body=[Expr(value=JoinedStr(values=[FormattedValue(value=Name(id='a', ctx=Load()), conversion=-1)]))], type_ignores=[])"

[pablogsal]

@lysnikolaou Does this need to wait until #149 is landed?

[lysnikolaou]

@pablogsal This can go already before #149, I think.

[pablogsal]

Yes!!