STYLE: UseSlicer - Check variable only if required

Before this commit all three variables

• EXTENSION_NAME
• MODULE_NAME
• PROJECT_NAME
  where checked even if EXTENSION_NAME was already set. In case
  no "project()" call was done in the extension/module CMakeLists.txt,
  the associated output was confusing.

For example:

[...]
-- Checking EXTENSION_NAME variable
-- Checking EXTENSION_NAME variable - SkullStripper
-- Checking MODULE_NAME variable
-- Checking MODULE_NAME variable - SkullStripper
-- Checking PROJECT_NAME variable
-- Checking PROJECT_NAME variable - Project
[...]

This commit change the logic so that the "checking" message is
displayed only if the associated variable is either empty or not
defined.

For example, the output is now:

[...]
-- Checking EXTENSION_NAME variable
-- Checking EXTENSION_NAME variable - SkullStripper
[...]

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22064 3bd1e089-480b-0410-8dfb-8563597acbee