BUG: midas_api_upload_extension: Do not report warning if metadata are empty

This commit is a follow up of r25434 (BUG: Extension upload: Ensure
missing optional metatdata do not cause failure), it fixes the following
warning:

8: CMake Warning (dev) at /home/jcfr/Projects/Slicer/Extensions/CMake/MIDASAPIUploadExtension.cmake:75 (message):
8:   warning: CMake variable EXTENSION_CATEGORY is empty !
8: Call Stack (most recent call first):
8:   /home/jcfr/Projects/Slicer/Extensions/CMake/SlicerExtensionPackageAndUploadTarget.cmake:260 (midas_api_upload_extension)
8:
8: This warning is for project developers.  Use -Wno-dev to suppress it.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25437 3bd1e089-480b-0410-8dfb-8563597acbee