eval: Fix coverity false positive.

** CID 74786:  Resource leak  (RESOURCE_LEAK)
/src/nvim/eval.c: 10614 in f_jobsend()
/src/nvim/eval.c: 10616 in f_jobsend()

save_tv_as_string() should return NULL and input_len <= 0 for an empty
string or error. Callers should check that input != NULL instead of
input_len > 0 and assert(input == NULL) when the length must be checked.
Merge pull request #1566 from splinterofchaos/fix-save-tv-as-string

eval: Fix coverity false positive.