Json: Allow nil regex block argument

This updates the block-handling logic in `Json::versions_from_content`
to naively pass the regex value when the block has two parameters. Up
to now, we have been ensuring that `regex` is not `nil` and this
makes sense with existing usage (the `Crate` strategy's default
block, formulae/cask `strategy` blocks). However, we need to allow a
`nil` `regex` value to make it possible to add an optional `regex`
parameter in the `Pypi::DEFAULT_BLOCK` Proc. This is necessary to
allow the `Pypi` strategy to work with an optional regex from a
`livecheck` block again [without creating an additional
`DEFAULT_BLOCK` variant with a regex parameter].

Library/Homebrew/livecheck/strategy/json.rb

@@ -58,8 +58,10 @@ module Homebrew
         end
 
         # Parses JSON text and identifies versions using a `strategy` block.
-        # If a regex is provided, it will be passed as the second argument to
-        # the  `strategy` block (after the parsed JSON data).
+        # If the block has two parameters, the parsed JSON data will be used as
+        # the first argument and the regex (if any) will be the second.
+        # Otherwise, only the parsed JSON data will be passed to the block.
+        #
         # @param content [String] the JSON text to parse and check
         # @param regex [Regexp, nil] a regex used for matching versions in the
         #   content
@@ -77,10 +79,8 @@ module Homebrew
           json = parse_json(content)
           return [] if json.blank?
 
-          block_return_value = if regex.present?
+          block_return_value = if block.arity == 2
             yield(json, regex)
-          elsif block.arity == 2
-            raise "Two arguments found in `strategy` block but no regex provided."
           else
             yield(json)
           end

Library/Homebrew/test/livecheck/strategy/json_spec.rb

@@ -107,11 +107,6 @@ RSpec.describe Homebrew::Livecheck::Strategy::Json do
       expect(json.versions_from_content(content_simple, regex) { next }).to eq([])
     end
 
-    it "errors if a block uses two arguments but a regex is not given" do
-      expect { json.versions_from_content(content_simple) { |json, regex| json["version"][regex, 1] } }
-        .to raise_error("Two arguments found in `strategy` block but no regex provided.")
-    end
-
     it "errors on an invalid return type from a block" do
       expect { json.versions_from_content(content_simple, regex) { 123 } }
         .to raise_error(TypeError, Homebrew::Livecheck::Strategy::INVALID_BLOCK_RETURN_VALUE_MSG)