Allow empty properties in YAML metadata

In YAML metadata you can specify properties without a value. For example:

number: 3
empty:
string: Hello

In this example, property "empty" does not have a value. When this YAML snippet is parsed by Ruby, that property "empty" gets value nil. In some situations, pandocomatic would crash when it encountered this nil value for a property because pandocomatic did expect all properties to have a value.

From pandocomatic 2.8 onwards, empty properties are properly supported: Pandocomatic now skips empty properties.

This change should not affect working pandocomatic setups.