Don't attempt to detect version if URL is not set

Library/Homebrew/resource.rb

@@ -148,6 +148,8 @@ class Resource
   private
 
   def detect_version(val)
+    return if val.nil? && url.nil?
+
     case val
     when nil     then Version.detect(url, specs)
     when String  then Version.new(val)

Library/Homebrew/test/test_resource.rb

@@ -81,6 +81,10 @@ class ResourceTests < Homebrew::TestCase
     assert_raises(TypeError) { @resource.version(Object.new) }
   end
 
+  def test_version_when_url_is_not_set
+    assert_nil @resource.version
+  end
+
   def test_mirrors
     assert_empty @resource.mirrors
     @resource.mirror('foo')