twister: Fix dependency on west when checking for projects

Twister should be partially usable even without West installed, however,
Twister unconditionally tries to import it when initializing to check
for west projects and crashes if it is not there.

This PR changes the behavior to return None if the west imports failed,
which appears to be more in line with what was intended. This allows
Twister to proceed and successfully run tests in a non-west environment.
There is no impact on environments that do have west installed.

Fixes #45355

Signed-off-by: Tristan Honscheid <honscheid@google.com>