Improve handling of null insertion failure for TreeSet

[cowtowncoder]

Describe your Issue

(note: tackling issue surfaced by OSS-Fuzz via https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=64509)

Currently attempts to deserialize null values for TreeSet will fail with underlying NPE which usually gets wrapped as JsonMappingException (unless DeserializationFeature.WRAP_EXCEPTIONS is disabled). While acceptable this is not optimal failure indication: let's instead catch NPE and throw more meaningful exception; similar to Guava change:

FasterXML/jackson-datatypes-collections#132

[cowtowncoder]

Note: change needed for CollectionDeserializer (and should only catch and handle NPE for TreeSet); we don't want to assume all NPEs are due to same root cause.

NOTE: earlier comment referred to TreeMap too -- but that might not have same issue (as there's no way to get null for JSON Object keys).