IMHO, it's a little too vague, why can't MASTER_SITES points to webarchive.org or any other permanent archive ?
Hmm. In some cases ports other than meta-ports can have MASTER_SITES left blank, like https://svnweb.freebsd.org/ports/head/www/googlebook_dl/Makefile?annotate=354597#l7
I would make that two sentences, it makes the sentences easier to read and understand:
Agreed on both. For the second sentence:
For example, meta-ports do not have any distribution files, so <varname>MASTER_SITES</varname> does not need to be set.