Populate marker environment with extra as "" by default

[pradyunsg]

IMO, packaging should assume an empty list of extras by default, rather than requiring the caller to specify it explicitly

Originally posted by @pfmoore in pypa/pip#11112 (comment)

I agree with @pfmoore. :)

[MrMino]

I believe this requires adding an empty string under the right key in markers.default_environment? Seems a bit too simple, what am I missing?

[MrMino]

I believe that, with this change, the markers.UndefinedEnvironmentName becomes redundant. It's impossible to trigger it.

The default environment already contains all of the possible marker names (checked using the code below), and the grammar set out in the module via pyparsing makes it impossible to use any other. evaluate() doesn't seem to have a way of explicitly specifying undefined name in the environment either.

Should it be removed?

The code below loops through all possible "variable" names, and does not raise any exception:

from packaging.markers import Marker

names = [
    "implementation_version",
    "platform_python_implementation",
    "implementation_name",
    "python_full_version",
    "platform_release",
    "platform_version",
    "platform_machine",
    "platform_system",
    "python_version",
    "sys_platform",
    "os_name",
    "os.name",
    "sys.platform",
    "platform.version",
    "platform.machine",
    "platform.python_implementation",
    "python_implementation",
    "extra",
]
for name in names:
    Marker(f"{name} == '1.2.3'").evaluate()

Edit: funnily enough, I had to remove the last place where raise UndefinedEnvironmentName was, because of coverage. I don't see a way of creating a proper unit test that goes through that clause.