Set @datanames in teal if unspecified

[gogonzo]

This simplifies a code in teal a little bit.
Instead of using .teal_data_datanames function which checks if data@datanames is specified and either get data@datanames or ls(data@env),
here we set data@datanames if unspecified and always refer to data@datanames

What it changes is that in teal_transform_module when evaluated, data passed to its server, will always have datanames specified.