tools.environment_append raises if tries to unset variable which was never set

[SSE4]

after #4224, I may use the following code, for instance, to ensure variable is not set:

with environment_append({'CONAN_BASH_PATH': None}):
    pass

however, it raises if CONAN_BASH_PATH is not set (prior to the environment_append invocation):

Traceback (most recent call last):
  File "C:\bincrafters\conan\conans\test\unittests\client\tools\os_info\osinfo_test.py", line 39, in test_windows
    with environment_append(new_env):
  File "c:\users\sse4\appdata\local\programs\python\python36\lib\contextlib.py", line 81, in __enter__
    return next(self.gen)
  File "C:\bincrafters\conan\conans\client\tools\env.py", line 57, in environment_append
    os.environ.pop(var)
  File "c:\users\sse4\appdata\local\programs\python\python36\lib\_collections_abc.py", line 795, in pop
    value = self[key]
  File "c:\users\sse4\appdata\local\programs\python\python36\lib\os.py", line 669, in __getitem__
    raise KeyError(key) from None
KeyError: 'CONAN_BASH_PATH'

I would expect tools.environment_append to be no op in such case, otherwise, it requires additional logic to workaround this behavior.

To help us debug your issue please explain:

• I've read the CONTRIBUTING guide.
• I've specified the Conan version, operating system version and any tool that can be relevant.
• I've explained the steps to reproduce the error or the motivation/use case of the question/suggestion.