[release/7.0] Fix for some minor bugs around JSON mappings

[maumar]

This is a collection of small fixes for JSON column mapping, which is a major new feature in EF7. We're bringing them together because @maumar has been out on vacation and these are fixes for the minor issues discovered while he was out. We will bring any future fixes individually.

Descriptions

#29214 - Json column: constructor with optional enum-parameter throws ArgumentException

When producing shaper code for json nullable properties with converters we check if the converter handles null, if not we need to peek into the json value and only pass it thru converter if it's not null, and return null otherwise

#29217 - Json: nullable enums on entities mapped to json don't use enum-to-int converter, rather than string, like for non-nullable enums

In convention code unwrap nullable type when testing for enum properties that we then apply converter to string

#29225 - Json: projecting nullable scalar produces debug.assert in SqlExpression ctor

Unwrap nullable type from property before passing it as type to SqlExpression ctor

#29219 - Json: error when trying to materialize json entity with nullable property that is missing in the json string - should materialize the property as null instead

Modify ExtractJsonProperty method so that it used TryGetProperty when trying to extract nullable properties.

Customer impact

Fixes for paper cuts in the JSON column mapping feature.

How found

Customer reported, team testing, and writing samples.

Regression

No; new feature in EF7.

Testing

New testing added.

Risk

Low-risk fixes for polishing.

Fixes #29214
Fixes #29217
Fixes #29225
Fixes #29219