lib-sieve: sieve-script - Rely on sieve_script_unref(NULL) being a no-op

src/lib-sieve/sieve-binary.c

src/lib-sieve/sieve-script.c

src/lib-sieve/sieve-storage.c

src/managesieve/cmd-getscript.c

src/plugins/doveadm-sieve/doveadm-sieve-cmd-get.c

src/plugins/doveadm-sieve/doveadm-sieve-cmd-put.c

src/plugins/doveadm-sieve/doveadm-sieve-cmd-rename.c

src/plugins/doveadm-sieve/doveadm-sieve-sync.c

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

src/plugins/imapsieve/imap-sieve.c

src/plugins/lda-sieve/lda-sieve-plugin.c