dotenv: skip keys that the callback blanks, like env.Provider

[Shaisolaris]

dotenv.ParserEnv and ParserEnvWithValue are documented as making the parser "behave like the env.Provider". env.Provider ignores a variable when its callback returns an empty key ("If the returned variable name is an empty string (""), it is ignored altogether"). The dotenv parser stored the value under the "" key instead:

p := dotenv.ParserEnv("APP_", ".", func(k string) string {
	if k == "APP_SECRET" {
		return ""
	}
	return strings.ToLower(strings.TrimPrefix(k, "APP_"))
})
m, _ := p.Unmarshal([]byte("APP_HOST=localhost\nAPP_SECRET=hunter2\n"))
// map[:hunter2 host:localhost]
out, _ := p.Marshal(m)
// APP_HOST="localhost"
// APP_SECRET="hunter2"

Because reverseCB also recorded "" -> APP_SECRET, Marshal wrote the dropped variable back out under its original name.

This change skips the entry when the callback returns an empty key, matching env.Provider.

Testing: TestUnmarshalSkipsBlankedKeys in parsers/dotenv checks both Unmarshal and Marshal for the case above. It fails on master and passes with the change. go test . and go vet . in parsers/dotenv pass. I did not run the tests of the other modules. Go 1.25 only.

[knadh]

Thanks. Released parsers/dotenv/v1.1.3