[filters] Match empty metadata values

[ivancea]

Problem

• Filters such as [Invoice]=[] cannot find assets with blank metadata.
• Blank metadata can be stored as a missing key, null, an empty string, or an empty tag list.

Solution

• Match each datatype's blank representation when filtering with [].
• Keep non-empty metadata filters unchanged.
• This cannot distinguish never-set from cleared metadata because both appear as blank values in Kitsu.

[NicoPennec]

Thanks @ivancea! This looks good to me overall.

The only thing I would change is the number case:
[Budget]=[] still misses cleared number metadata, since '' skips the new missing/null branch and parseFloat('') is NaN, so normalizing blank once before the data_type dispatch would cover every type.