Notable fixes:
Fix `${+}` parameter expansion inside loops
`nv_getval()` function internally calls `nv_optimize()` and
optimizations outside `varsub()` function depend on it. Using
`nv_isnull()` instead of `nv_getval()` was causing broken behavior
with `${+}` parameter expansions inside loops.
Resolves: #70
1b7543ed5c