Fix NoMethodError when calling selector methods w/o anony config

[Tabby]

When calling selector_for? or anonymise_for! on a model with no anonymise block, instead of throwing a NoMethodError due to the anonymisation config being nil we now do the following:

• Return false when calling selector_for?, same as if calling valid_anonymisation?
• Raise ArgumentError when calling anonymise_for!, same as if calling anonymise!