Dashboard context returns invalid filter

[simianhacker]

When there are no fitlers or queries the dashboard context returns an invalid bool filter.

{
  must: [null], 
  must_not: []
}

It needs to check to make sure the filter are not null before they are pushed into the must and must_not arrays. This bug was introduced with #12624

This was originally reported with elastic/beats#4777

[simianhacker]

CC @alexfrancoeur

[lukasolson]

CC @Bargs