Empty / Null Filters params blows up

[PatKoperwas]

Passing an empty filters param causes Brita to raise an exception.

Version: 0.10.0

To Reproduce: Make a request like /api/my-resource?filters= or /api/my-resource?filters or /api/my-resource?filters[]

Result:

undefined method `[]' for nil:NilClass

brita (0.10.0) lib/brita/filter_validator.rb:33:in `has_custom_validation?'
brita (0.10.0) lib/brita/filter_validator.rb:21:in `block (2 levels) in build'
brita (0.10.0) lib/brita/filter_validator.rb:20:in `each'
brita (0.10.0) lib/brita/filter_validator.rb:20:in `block in build'
brita (0.10.0) lib/brita/filter_validator.rb:13:in `initialize'
brita (0.10.0) lib/brita/filter_validator.rb:13:in `new'
brita (0.10.0) lib/brita/filter_validator.rb:13:in `build'
brita (0.10.0) lib/brita.rb:39:in `filter_validator'
brita (0.10.0) lib/brita.rb:29:in `filters_valid?'