Improve dynamic filters

[dirkkul]

We currently allow to add multiple filters dynamically using Filter.all_of(..list..of..filters...).

However, users still need to handle the case that:

• there are no filters (list is empty)
• some of the filters are None

We should handle this case and:

• filter None out
• In case there are no filters, we return a Non-filter that works like any other of our filters (eg supports + and - with other filters). Internally they are then ignored