Fix nrpes var init

[sed-i]

Issue

When _on_nrpe_targets_changed is called with no event arg (event=None), the nrpes variable cannot be init'ed from it.

Solution

• Initialize the nrpes variable from stored state. If the event arg is None, then at this point the stored state value is already up to date.
• Add relevant tests

Fixes #92.

[PietroPasotti]

please add a test for this path though, so we don't have the same issue again :/

[sed-i]

Will merge and test manually with @przemeklal. Tests will be added after #93.
Captured in #95.