pygmt.grd2cpt & pygmt.makecpt: Simplify the logic for dealing with CPT output

[seisman]

Simplify:

        if kwargs.get("H") is None:  # if no output is set
            arg_str = build_arg_list(kwargs)
        else:  # if output is set
            outfile, kwargs["H"] = kwargs.pop("H"), True
            if not outfile or not isinstance(outfile, str):
                raise GMTInvalidInput("'output' should be a proper file name.")
            arg_str = build_arg_list(kwargs, outfile=outfile)

to

    if (output := kwargs.pop("H", None)) is not None:
        if not isinstance(output, str) or output == "":
            raise GMTInvalidInput("'output' should be a proper file name.")
        kwargs["H"] = True
    arg_str = build_arg_list(kwargs, outfile=output)

It works because outfile in build_arg_list can be None.

[weiji14]

Needed to read that if-statement a few times to get the logic, but looks ok.

[seisman]

The if-statement can be further simplified after PR #3238