Description

SchemaDiff had a single method checkTypeWithNonNullAndList that always applied the rule "non-null to nullable is safe; nullable to non-null is breaking."
This is true for input types and arguments, but the opposite should be true for fields on output types -- "nullable to non-null is safe; non-null to nullable is breaking."
A consumer operating on a nullable field should already be checking for presence, and adding an additional guarantee of non-null is safe.
A consumer operating on a non-null field may not be checking for presence, and removing the guarantee of non-null may break when that field is null.

Implementation

Two methods are defined -- one for input and argument types, and one for output types.
The methods are rewritten in a style of handling three top-level cases for the old type (non-null, list, and neither), and within each case handling the cases for the new type.
Calls to the previous method are updated to their respective new versions.

graphql-js implementation for reference:
isChangeSafeForObjectOrInterfaceField
isChangeSafeForInputObjectFieldOrFieldArg

Testing

Unit tests for the new methods are updated and added:

• change_in_null_ness_object_or_interface
• change_in_null_ness_input_or_arg
• change_in_list_ness_object_or_interface
• change_in_list_ness_input_or_arg

schema_ABaseLine:

• Added Query.allCharactersByTemperament : [[Character]] to test a complex wrapped type.
• Made Questor.beingID non-null to be able to test "non-null to nullable" for input.

schema_changed_input_object_fields:

• Added case Query.being( .. , newArg: String!) -> stricter (moved from schema_changed_object_fields to here)
• Added case Questor.beingID "required to optional" -> safe
• Added case Questor.nestedInput "optional to required" -> stricter

schema_changed_object_fields:

• Moved case Query.being( .. , newArg: String!) to schema_changed_object_fields
• Added case Query.allCharactersByTemperament : [[Character!]]! to test complex wrapped type "optional to required" -> safe
• Added case Being.ID "optional to required" -> safe
• Added case Istari.temperament "required to optional" -> stricter