Labels with empty values should not be listed or documented

[zanetworker]

Currently, there are no checks for empty values when creating the Dockument markdown for images and docker files, as a result, empty keys are still printed or documented, for example:

dockument tests -i "zanetworker/dockument"

Yields:

	Name: apt-get upgrade
	Command: apt-get
	Args: [-qqs, upgrade]
	Expected Ouput: true
	Expected Error:
	Excluded Ouput:
	Excluded Error:

Should be:

	Name: apt-get upgrade
	Command: apt-get
	Args: [-qqs, upgrade]
	Expected Ouput: true