Fix catch_discover_tests PRE_TEST failure with zero discoverable tests

When using catch_discover_tests() with DISCOVERY_MODE PRE_TEST and a
multi-config generator (e.g. Ninja Multi-Config), if a test target has
zero discoverable tests (e.g. all tests tagged with [.]), ctest fails:

  CMake Error: include could not find requested file:
    .../test-hidden-b12d07c_tests-Release.cmake

The early return added in #2962 (76f70b14) correctly prevented a JSON
parsing crash for zero tests, but skipped writing the ctest file. The
PRE_TEST include script unconditionally includes this file, so the
missing file causes a hard error that aborts all test discovery.

Write an empty file before returning early so the include always
succeeds.

extras/CatchAddTests.cmake

@@ -146,6 +146,7 @@ function(catch_discover_tests_impl)
 
   # Exit early if no tests are detected
   if(num_tests STREQUAL "0")
+    file(WRITE "${_CTEST_FILE}" "")
     return()
   endif()