Guava collection types do not allow null values

[josethomas]

ImmutableMap (and other types) do not allow null values. Trying to store nulls will result in a java.lang.NullPointerException. Can we simply eliminate storing nulls? If this PR acceptable, I can make similar changes for other types as well.

[cowtowncoder]

Hmmh. This is worth asking on dev list. I agree that getting an NPE is not optimal; but on the other hand, quietly changing the contents can also lead to subtle problems... So perhaps there should be choice, perhaps a module feature to choose between explicit exception that explains the problem, and elimination of nulls?

[cowtowncoder]

FWIW, I think that

1. Yes, quietly avoiding nulls for collections that can not store them is the right default behavior, and
2. It would be great to have a feature to alternatively allow an exception other than NPE to be thrown, instead of quiet swallowing

[ankitcha]

+1 to both points

[cowtowncoder]

@josethomas I can merge this, makes sense.

One practical thing we need before merge is to get a filled Contributor Licenses Agreement (CLA), see contributor-agreement.pdf

The usual way is to print it, fill and sign, scan and email to info at fasterxml dot com.
If you could do this, I can merge the patch for inclusion in 2.6.2.
We only need CLA for the first contribution, after which we can accept contributions for all Jackson projects.

[josethomas]

CLA sent.