Allow FiltersQParser (`{!filters param=[...]}`) to tolerate empty `param` args

One common use case for FiltersQParser is to bundle filters and main query, e.g. for passing a single-query domain definition to functions, or for finer-grained control over how logical filters are bundled together.

Consider the first use case: `{!filters param=$q param=$fq}`

FiltersQParser currently strictly enforces that the referred params must be present and non-empty, and fails hard (throws a SyntaxError) if this is not the case.

This issue proposes to relax this enforcement, because:

1. There are legitimate, common use cases for which this requirement is simply an inconvenience (albeit easily worked around by adding a "dummy" fq=*:*), and
2. It's unclear what practical purpose is served by by the current strict enforcement of non-empty params.

The proposal is to have e.g. `{!filters param=$fq}`, in the absence of any non-empty fq param, be equivalent to *:*.