Improve collection ergonomics and add Base#empty? default semantics

[mjgiarlo]

This PR improves day-to-day form-object ergonomics by expanding collection behavior on Blanks::AssociationProxy and adding default empty? semantics to Blanks::Base.

• Kept association emptiness behavior (AssociationProxy#empty?) as a direct core API and documented adjacent collection ergonomics.
• Updated AssociationProxy#any? to support both no-block and block forms.
• Added AssociationProxy#reverse to return records in reverse order.
• Added Base#empty? with default semantics: all attribute values are blank.
• Added spec coverage for the above changes.
• Updated README with collection proxy and empty? usage notes.
• Added CHANGELOG entries under Unreleased.

Apps using Blanks often treat has_many associations as collections. Without block-aware any? and reverse, callers must unwrap via to_a for routine operations.

Likewise, many applications define identical empty? methods in local base forms. Moving this to Blanks::Base provides a shared default and reduces boilerplate.

• AssociationProxy#any?:

  • proxy.any? works as before.
  • proxy.any? { |item| ... } now evaluates the block against members.

• AssociationProxy#reverse:

  • Returns an array in reverse order.
  • Does not mutate the proxy's underlying order.

• Base#empty?:

  • Returns true when all attributes values are blank.
  • Returns false when any attribute is present.
  • Subclasses may still override for custom emptiness semantics.

• Backward compatible for existing any? calls without a block.

• to_a remains available and unchanged.

• Adding Base#empty? may affect classes that relied on NoMethodError, but aligns with common form-object expectations.