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