lib-sieve: sieve-storage - Make sieve_storage_unref(NULL) a no-op

src/lib-sieve/plugins/include/ext-include-common.c

src/lib-sieve/sieve-storage.c

src/lib-sieve/storage/file/sieve-file-storage.c

src/plugins/imap-filter-sieve/imap-filter-sieve.c

src/plugins/imapsieve/imap-sieve.c