Skip empty value assignment

[ezh]

There might be a situation when a shell contains an empty environment variable. In such a case, we get the next error for a boolean field, for example:

strconv.ParseBool: parsing "": invalid syntax

I propose to treat empty string values as absent ones.

[ezh]

Hey @ilyakaznacheev 👋 Is it possible to apply this fix? IMO it keeps logic the same. This is a very small improvement.

[ilyakaznacheev]

Well, I'm not sure that it is what we want. IMO the user may want to set to the variable an empty value or space - and the change will break that case.

I agree with the case with the boolean field - it is indeed an error and needs to be fixed. But maybe we only need to do this validation when we process boolean fields.