make sure values not provided by users resolve to undefined

[softprops]

addresses at least part of #142 #140 and #139

to summarize, after much trial and error I discovered an interesting quirk with the actions runtime env. all inputs (with values) are exposed to actions as env variables of the form INPUT_${inputname.toUpperCase()} even when users don't provide them which is not what you might expect from process.env

the actions runtime provides these instead as present with empty strings. since we use this as rest api inputs and value with an empty string != the absence of a value and hence things did not go as expected

I realized this when running experiments in a test repo using debug branch of this action and printed the param structs before requesting info from the api.

this theory also lines up with #139 (comment)

I was able to validate this pulls change on that branch with this test

https://github.com/softprops/action-gh-releases-test/actions/runs/1115229438

note: there are still a few quirks left over from recent changes like duplicate content of release bodies (see #140 (comment)) but I'd like to get this, the most significant change in first