Do not mock on nil.

[#47979793]

Using `should_not_receive` on `nil` gives `nil` a permanent
`@mock_proxy`, which causes Marshal.dump to fail in later tests (see
https://travis-ci.org/rapid7/metasploit-framework/builds/6502350).  By,
checking there are no NoMethoErrors raised, nil can be tested as
parent_module, but works around the RSpec issue
(https://github.com/rspec/rspec-mocks/issues/274).