Support coercion of empty strings to null objects

[robmoore]

We're working with a set of files that contains both empty strings and NULL values. We are using .withNullValue("NULL") as well as DeserializationFeature.ACCEPT_EMPTY_STRING_AS_NULL_OBJECT but are ending up with empty strings rather than null values for these cases.

In issue 112, @cowtowncoder suggested it might be possible to support an alternate notation in addition to the explicit null value indicated in withNullValue().

[cowtowncoder]

Moved: FasterXML/jackson-dataformats-text#7