fix: envPrefix should not contain spaces

[btea]

If the value of envPrefix contains only spaces, it should probably be treated as an empty string.

[btea]

Based on the test cases, it seems that strings containing only spaces are allowed.

However, simple spaces seem meaningless; do we need to handle them specially? 🤔

[bluwy]

I think we can remove that edge case in the tests. It doesn't make much sense since you can't use spaces in env vars the command line, or in an .env file. The only way is to set in process.env[" spaces "] manually but this also feels very rare to support for me.