Add support for specifying an override for converting null for String values to empty String

[cowtowncoder]

There are some cases where a specific coercion from null token (JsonToken.VALUE_NULL) into empty String ("") would make sense. One example is when processing XML input, where there is no low-level distinction between empty String and null; and since parser implementation does not have enough context to determine expected type, it simply returns null as token.
At higher level, however, it is important to be able to coerce this into empty String.
For more background, see FasterXML/jackson-dataformat-xml#162.

This could be achieved quite easily via StringDeserializer implementation by return empty String from getNullValue(). However: since such coercion should not be done in cases where null comes from native null type (or is otherwise unambiguous), some mechanism is needed to distinguish between cases where this is and is not desired.

The are many possible ways to pass/access this information, including contextualization of StringDeserializer, or even just dynamically from within getNullValue(), so I will leave the exact mechanism open.

One thing to check is Afterburner, which can override standard String handling: it should not change behavior of "null-to-Empty-String" coercion.

[cowtowncoder]

Sort of related to #1402; but not quite the same since this would probably operate at lower level.

[cowtowncoder]

Note: with #1402, it is actually possible to convert nulls into empty String with:

public class Bean {
   @JsonSetter(nulls=JsonSetter.Nulls.AS_EMPTY)
   public String neverNullString = "";
}

so it is not clear whether this feature is needed or not, after all.

[cowtowncoder]

Actually I think #1402 might do for now; will re-file if I missed something obvious.