Update signature handling in @overload

Previously, `@overload` tags that defined required keyword arguments would end up with an empty default value, to the delight of nobody. While this might *ideally* be handled by moving away from the legacy parsers, this is a stopgap that makes an already uncommon edge case go away.

lib/yard/tags/overload_tag.rb

@@ -60,7 +60,8 @@ def parse_signature
           args = YARD::Handlers::Ruby::Legacy::Base.new(nil, nil).send(:tokval_list, toks, :all)
           args = args.map do |a|
             k, v = *a.split(/:|=/, 2)
-            [k.strip.to_s + (a[k.size, 1] == ':' ? ':' : ''), (v ? v.strip : nil)]
+            v.strip! if v
+            [k.strip.to_s + (a[k.size, 1] == ':' ? ':' : ''), (v && v.empty? ? nil : v)]
           end if args
           @name = meth.to_sym
           @parameters = args

spec/tags/overload_tag_spec.rb

@@ -50,4 +50,19 @@ def bar(a, b = 1, &block)
     tag = Tags::OverloadTag.new(:overload, "default")
     expect(tag.signature).to eq "default"
   end
+
+  it "properly handles complex signatures" do
+    tag = Tags::OverloadTag.new(:overload, "foo(a, b = 1, *c, d, e:, f: 2, g:, **rest, &block)")
+    expect(tag.parameters).to eq [
+      ['a', nil],
+      ['b', "1"],
+      ['*c', nil],
+      ['d', nil],
+      ['e:', nil],
+      ['f:', "2"],
+      ['g:', nil],
+      ['**rest', nil],
+      ['&block', nil],
+    ]
+  end
 end