Protect filter() against damaged projects

[ferdnyc]

In #3171 a user uploaded logs that contained a bunch of these errors:

 main_window:INFO main::addSelection: item_id: , item_type: effect, clear_existing: True
  exceptions:ERROR Unhandled Exception
Traceback (most recent call last):
  File "/usr/lib/python3.6/site-packages/openshot_qt/windows/views/properties_tableview.py", line 699, in select_item
    clip = Clip.get(id=self.item_id)
  File "/usr/lib/python3.6/site-packages/openshot_qt/classes/query.py", line 162, in get
    return QueryObject.get(Clip, **kwargs)
  File "/usr/lib/python3.6/site-packages/openshot_qt/classes/query.py", line 135, in get
    matching_objects = QueryObject.filter(OBJECT_TYPE, **kwargs)
  File "/usr/lib/python3.6/site-packages/openshot_qt/classes/query.py", line 110, in filter
    if key in child and not child[key] == value:
TypeError: argument of type 'NoneType' is not iterable

It doesn't appear they're fatal, though they may have led to the subsequent project-loading issues experienced. Regardless, it's a good idea to protect against them. This should.

The unit tests still pass, so that should hopefully be a good sign that it at least doesn't make things worse.

[SuslikV]

The log import part

openshot-qt/src/classes/query.py

Line 33 in f4c3aa6

from classes.logger import log

not required, I think.

[ferdnyc]

Whoops, did I leave that in? Quite right, thanks. Fixed.