This changes the check for null/undefined to a check for undefined to determine if scalar serialization was successful or not, allowing `null` to be returned from serialize() without indicating error.
This is potentially breaking for any existing custom scalar which returned `null` from `serialize()` to indicate failure. To account for this change, it should either throw an error or return `undefined`.
ref: graphql/graphql-js#1104
* Generalizes building a value from an AST, since "scalar" could be misleading, and supporting variable values within custom scalar literals can be valuable.
* Replaces isNullish with isInvalid since `null` is a meaningful value as a result of literal parsing.
* Provide reasonable default version of 'parseLiteral'
ref: 714ee980aa
ref: https://github.com/graphql/graphql-js/pull/903
# Conflicts:
# src/Utils/BuildSchema.php
# tests/Utils/BuildSchemaTest.php
