src/helper.cpp - fix crash in setResourceColor

don't crash on empty collection ColorAttribute

BUG: 426906
FIXED-IN: 21.08.2
1 job for release/21.08 in 3 minutes and 18 seconds (queued for 58 minutes and 54 seconds)
Status Name Job ID Coverage
failed build_clazy_clang_tidy #131345


Name Stage Failure
build_clazy_clang_tidy Build
1 warning and 3 errors generated.
make[2]: *** [src/CMakeFiles/KF5EventViews.dir/build.make:215: src/CMakeFiles/KF5EventViews.dir/agenda/calendardecoration.cpp.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:217: src/CMakeFiles/KF5EventViews.dir/all] Error 2
make: *** [Makefile:146: all] Error 2
Uploading artifacts for failed job
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ERROR: No files to upload
Cleaning up file based variables
ERROR: Job failed: exit code 1