(maint) Fix typo in comment
(PUP-2714) Support explicit creation of empty program

An empty program (one without any expressions except "no-op") did
not produce an instance of Model::Program. This is bad because it
is of value to get the lexed/parsed model with line references for
comment processing and other static analysis of the source.

Now, an empty program will produce a Model::Program containing a single
Model::Nop instruction. This instruction has offset at "EOF", and a
length of 0 (thus, any comments in the input source are guaranteed to be
placed before this nop.
Merge pull request #2749 from hlindberg/PUP-2714_handle-empty-program

(PUP-2714) handle empty program