feat(flags,command): skip optional options and arguments that have an empty string as value

[c4spar]

resolves #731
fixes #665

This pull request improves the handling of optional arguments and options.

Optional arguments and options with an empty string as their value are now skipped and set to undefined.

Assuming that the option --foo is an optional option, the command my-cli --foo="" --bar="123" would be equivalent to my-cli --bar="123".

This makes it easier to use commands in scripts that accept conditional arguments without having to parse them manually.

Example: If the following command is executed: my-cli --optional-option "$MY_ENV_VAR" and MY_ENV_VAR is not defined, the option --optional-option is ignored and treated as not defined.

This also applies to arguments. Arguments defined as follows [foo] [bar] [...baz] and called as follows my-cli "foo-value" "" "baz-value-1" "" "baz-value-3", are parsed to "foo-value", undefined, "baz-value-1", "baz-value-3"`.

Single arguments with a "" as their value are set to undefined, and rest arguments with a "" as their value are ignored.