ci: Make `rspec:coverage` gracefully exit with no coverage file

Signed-off-by: Rémy Coutable <remy@rymai.me>

scripts/merge-simplecov

@@ -5,4 +5,7 @@ require_relative '../spec/simplecov_env'
 SimpleCovEnv.configure_profile
 SimpleCovEnv.configure_formatter
 
-SimpleCov.collate Dir.glob(File.join(SimpleCov.coverage_path, '*', '.resultset.json'))
+resultset_files = Dir.glob(File.join(SimpleCov.coverage_path, '*', '.resultset.json'))
+exit(0) if resultset_files.empty?
+
+SimpleCov.collate(resultset_files)