Gix missing checks task doesn't go back enough

When a new channel is created, last month empty checks are created for this channel by fix_missing_checks task. But fix missing checks should create empty checks from the beginning of the network monitored at least.

This bug has been introduced by this commit 9b97d7ce related to issue #183 (closed)

For now we should just revert this commit and think later on how to fix the slow db issue

Sentry Issue: WAVEQC-1F

ValueError: setting an array element with a sequence. The requested array has an inhomogeneous shape after 1 dimensions. The detected shape was (183,) + inhomogeneous part.
(11 additional frame(s) were not displayed)
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  File "waveqc/views.py", line 433, in network_detail
    return self.context(query, label, pk, items)
  File "waveqc/views.py", line 355, in context
    results=self.build_z_values(query),
  File "waveqc/views.py", line 244, in build_z_values
    return np.array(