Fix NPE in writeNumber(String) method of UTF8JsonGenerator, WriterBasedJsonGenerator

[pakru]

I faced NPE in writeNumber(String) of UTF8JsonGenerator when trying to pass null into this method.

gen.writeNumber(getStringValue());
where getStringValue() method can return null, and this causes NPE in writeRaw(String text) method of generator.
But it works fine for writeNumber(BigDecimal) and writeNumber(BigInteger) methods if I pass null into those methods.

Thus, I added null check for writeNumber(String encodedValue) the same as it made for BigDecimal and BigInteger types. Furthermore modified some tests for this nullability check.

[cowtowncoder]

Looks legit, thank you! (and especial thanks for improving tests).

One practical thing before I merge this: unless I have asked for and received CLA (only needs to be done once, good for all PRs for Jackson repositories), I'd need this:

https://github.com/FasterXML/jackson/blob/master/contributor-agreement.pdf

and usually simplest way is to print, fill & sign, scan/photo, email to info at fasterxml dot com.
Once I receive it, we are good to go!

Thank you once again for your contribution!

[pakru]

Thanks for your feedback!
I have sent you signed CLA. Check you email, please.

[cowtowncoder]

Thanks! Received, will merge.