[CI] Do not fail with no JUnit XML

Currently we will fail if there are no JUnit XML files produced from
llvm-lit invocations. This can happen if the build fails and no test
suites end up getting run or if we test a project that does not use
llvm-lit, libe libc.

This fixes llvm#142038.

Author: boomanaiden154

.ci/monolithic-linux.sh

@@ -38,7 +38,7 @@ function at-exit {
 
   ccache --print-stats > artifacts/ccache_stats.txt
   cp "${BUILD_DIR}"/.ninja_log artifacts/.ninja_log
-  cp "${BUILD_DIR}"/test-results.*.xml artifacts/
+  cp "${BUILD_DIR}"/test-results.*.xml artifacts/ || :
 
   # If building fails there will be no results files.
   shopt -s nullglob

.ci/monolithic-windows.sh

@@ -33,7 +33,7 @@ function at-exit {
   mkdir -p artifacts
   sccache --show-stats >> artifacts/sccache_stats.txt
   cp "${BUILD_DIR}"/.ninja_log artifacts/.ninja_log
-  cp "${BUILD_DIR}"/test-results.*.xml artifacts/
+  cp "${BUILD_DIR}"/test-results.*.xml artifacts/ || :
 
   # If building fails there will be no results files.
   shopt -s nullglob