Improve support for cases with empty --path provided to wp-cli commands

[wojtekn]

I propose fixing the confusing messaging we show if an empty --path parameter is provided to command calls.

The empty parameter is considered a true flag, let's treat it as empty path:

% bin/wp import --path
Error: The --path parameter cannot be empty when it is provided

This is also an empty parameter considered a true flag because there is no equal sign used to assign a path to the parameter. Let's treat it as an empty path too:

% bin/wp import --path some
Error: The --path parameter cannot be empty when it is provided

Then handle an empty string as an empty path:

% bin/wp import --path=
Error: The --path parameter cannot be empty when it is provided

When a non-empty path is provided, include the full working directory in the error message:

% bin/wp import --path=some
Error: This (/Volumes/Sites/wp-cli/some/) does not seem to be a WordPress installation.
Pass --path=`path/to/wordpress` or run `wp core download`.

Fixes: #5559

[danielbachhuber]

Thanks @wojtekn !

Can you write some functional tests for this change?

The handbook is editable if there are improvements you identify.

[wojtekn]

@danielbachhuber that is good idea, I've added functional tests for three cases.

[danielbachhuber]

Looks good to me 👍