NPE when visiting StaticListSerializerBase

[WorldSEnder]

I am using jackson 2.7 but when looking through the code, the issue will most likely remain after upgrading.

The problem has it's roots here

jackson-databind/src/main/java/com/fasterxml/jackson/databind/ser/std/StaticListSerializerBase.java

Lines 94 to 96 in 54dc162

	public void acceptJsonFormatVisitor(JsonFormatVisitorWrapper visitor, JavaType typeHint) throws JsonMappingException {
	acceptContentVisitor(visitor.expectArrayFormat(typeHint));
	}

If the visitor returns null from expectArrayFormat (as for example by inheriting from JsonFormatVisitorWrapper.Base) then a NullPointerException will be raised from the following places

jackson-databind/src/main/java/com/fasterxml/jackson/databind/ser/impl/IndexedStringListSerializer.java

Lines 53 to 55 in db69efd

	protected void acceptContentVisitor(JsonArrayFormatVisitor visitor) throws JsonMappingException {
	visitor.itemsFormat(JsonFormatTypes.STRING);
	}

jackson-databind/src/main/java/com/fasterxml/jackson/databind/ser/impl/StringCollectionSerializer.java

Lines 55 to 58 in db69efd

	protected void acceptContentVisitor(JsonArrayFormatVisitor visitor) throws JsonMappingException
	{
	visitor.itemsFormat(JsonFormatTypes.STRING);
	}

.

[cowtowncoder]

It would be great to have a unit test to reproduce this (and guard against regression), but I agree that compared to other handlers this probably should check for nulls.