Ignore empty patch blocks

[Rylan12]

In order to make formulae with on_{system} blocks easier to read, we'd like to keep patches and resources where they're supposed to go (i.e. below dependencies) even if they are only present on some system configurations. However, since only one of each on_{system} block is allowed, we need to next the on_{system} block within the patch or resource block as is done in this example:

patch do
  on_high_sierra :or_newer do
    url "https://raw.githubusercontent.com/Homebrew/formula-patches/eed63e836e/alure/unistd.patch"
    sha256 "7852a7a365f518b12a1afd763a6a80ece88ac7aeea3c9023aa6c1fe46ac5a1ae"
  end
end

When the condition isn't met (in this example when MacOS.version < :high_sierra), this is the same as an empty patch do; end block which is not currently allowed. This PR changes the login of SoftwareSpec#patch to ignore any external patches that don't define a URL.

This was originally done in #13539 but needed to be extracted.

[BrewTestBot]

Review period skipped due to critical label.