Remove nil check in exec_byte_code

Remove nil check in exec_byte_code

Since we pass no arguments to a non-lexbind bytecode function, we can
specify its arity as 0 instead of nil and save a test and branch.

* src/bytecode.c (Fbyte_code, exec_byte_code):
* src/eval.c (fetch_and_exec_byte_code, funcall_lambda):
* src/lisp.h:
Change the args_template parameter type to ptrdiff_t, since it is now
always a small integer, in exec_byte_code and
fetch_and_exec_byte_code, all callers adjusted.