lib-sieve: adjusted code fetch api for handling omitted operands better.

src/lib-sieve/cmd-redirect.c

src/lib-sieve/ext-fileinto.c

src/lib-sieve/plugins/date/tst-date.c

src/lib-sieve/plugins/editheader/cmd-deleteheader.c

src/lib-sieve/plugins/imap4flags/tag-flags.c

src/lib-sieve/sieve-code.c

src/lib-sieve/sieve-code.h