os/mac/sdk: refactor for readability.

Library/Homebrew/os/mac/sdk.rb

@@ -52,6 +52,8 @@ module OS
         rescue NoSDKError
           latest_sdk
         end
+        return if sdk.blank?
+
         # Accept an SDK for another OS version if it shares a major version
         # with the current OS - for example, the 11.0 SDK on 11.1,
         # or vice versa.
@@ -59,11 +61,11 @@ module OS
         # or greater, given the way the versioning has changed.
         # This shortcuts the below check, since we *do* accept an older version
         # on macOS 11 or greater if the major version matches.
-        return sdk if sdk.present? && OS::Mac.version >= :big_sur && sdk.version.major == OS::Mac.version.major
+        return sdk if OS::Mac.version >= :big_sur && sdk.version.major == OS::Mac.version.major
 
         # On OSs lower than 11, or where the major versions don't match,
         # only return an SDK older than the OS version if it was specifically requested
-        return unless v || (sdk.present? && sdk.version >= OS::Mac.sdk_version)
+        return if v.blank? && sdk.version < OS::Mac.version
 
         sdk
       end