shared: handle missing Xcode/CLT version.

Fixes https://github.com/Homebrew/homebrew-core/issues/3973.

Library/Homebrew/extend/os/mac/extend/ENV/shared.rb

 module SharedEnvExtension
   def no_weak_imports?
     return false unless compiler == :clang
-    MacOS::Xcode.version >= "8.0" || MacOS::CLT.version >= "8.0"
+
+    if MacOS::Xcode.version && MacOS::Xcode.version < "8.0"
+      return false
+    end
+
+    if MacOS::CLT.version && MacOS::CLT.version < "8.0"
+      return false
+    end
+
+    true
   end
 end