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SPEC/BUG: Ambiguity with null variable values and default values

This change corresponds to a spec proposal which solves an ambiguity in how variable values and default values behave with explicit null values. Otherwise, this ambiguity allows for null values to appear in non-null argument values, which may result in unforseen null-pointer-errors.

This appears in three distinct but related issues:

**VariablesInAllowedPosition validation rule**

The explicit value `null` may be used as a default value for a variable, however `VariablesInAllowedPositions` allowed a nullable type with a default value to flow into an argument expecting a non-null type. This validation rule must explicitly not allow `null` default values to flow in this manner.

**Coercing Variable Values**

coerceVariableValues allows the explicit `null` value to be used over a default value, which can result in flowing a null value to a non-null argument when paired with the validation rule mentioned above. Instead a default value must be used even when an explicit `null` value is provided.

**Coercing Argument Values**

coerceArgumentValues allows the explicit `null` default value to be used before checking for a non-null type. This could inadvertently allow a null value into a non-null typed argument.

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