[JsonSerializer] Verify null can't be assigned to non-nullable structs

[layomia]

Closes#1037. Fixed during serializer refactoring - #2259.

We have additional tests for this scenario:

runtime/src/libraries/System.Text.Json/tests/Serialization/ConstructorTests/ConstructorTests.ParameterMatching.cs

Lines 342 to 370 in 0f7370b

	// Throw JsonException when null applied to types that can't be null. Behavior should align with properties deserialized with setters.
	awaitAssert.ThrowsAsync<JsonException>(()=>Serializer.DeserializeWrapper<NullArgTester>(@"{""Point3DStruct"":null,""Int"":null,""ImmutableArray"":null}"));
	awaitAssert.ThrowsAsync<JsonException>(()=>Serializer.DeserializeWrapper<NullArgTester_Mutable>(@"{""Point3DStruct"":null,""Int"":null,""ImmutableArray"":null}"));
	awaitAssert.ThrowsAsync<JsonException>(()=>Serializer.DeserializeWrapper<NullArgTester>(@"{""Point3DStruct"":null}"));
	awaitAssert.ThrowsAsync<JsonException>(()=>Serializer.DeserializeWrapper<NullArgTester_Mutable>(@"{""Point3DStruct"":null}"));
	awaitAssert.ThrowsAsync<JsonException>(()=>Serializer.DeserializeWrapper<NullArgTester>(@"{""Int"":null}"));
	awaitAssert.ThrowsAsync<JsonException>(()=>Serializer.DeserializeWrapper<NullArgTester_Mutable>(@"{""Int"":null}"));
	awaitAssert.ThrowsAsync<JsonException>(()=>Serializer.DeserializeWrapper<NullArgTester>(@"{""ImmutableArray"":null}"));
	awaitAssert.ThrowsAsync<JsonException>(()=>Serializer.DeserializeWrapper<NullArgTester_Mutable>(@"{""ImmutableArray"":null}"));
	// Throw even when IgnoreNullValues is true for symmetry with property deserialization,
	// until https://github.com/dotnet/runtime/issues/30795 is addressed.
	varoptions=newJsonSerializerOptions{IgnoreNullValues=true};
	awaitAssert.ThrowsAsync<JsonException>(()=>Serializer.DeserializeWrapper<NullArgTester>(@"{""Point3DStruct"":null,""Int"":null,""ImmutableArray"":null}",options));
	awaitAssert.ThrowsAsync<JsonException>(()=>Serializer.DeserializeWrapper<NullArgTester_Mutable>(@"{""Point3DStruct"":null,""Int"":null,""ImmutableArray"":null}",options));
	awaitAssert.ThrowsAsync<JsonException>(()=>Serializer.DeserializeWrapper<NullArgTester>(@"{""Point3DStruct"":null}",options));
	awaitAssert.ThrowsAsync<JsonException>(()=>Serializer.DeserializeWrapper<NullArgTester_Mutable>(@"{""Point3DStruct"":null,""Int"":null,""ImmutableArray"":null}",options));
	awaitAssert.ThrowsAsync<JsonException>(()=>Serializer.DeserializeWrapper<NullArgTester>(@"{""Int"":null}",options));
	awaitAssert.ThrowsAsync<JsonException>(()=>Serializer.DeserializeWrapper<NullArgTester_Mutable>(@"{""Point3DStruct"":null,""Int"":null,""ImmutableArray"":null}",options));
	awaitAssert.ThrowsAsync<JsonException>(()=>Serializer.DeserializeWrapper<NullArgTester>(@"{""ImmutableArray"":null}",options));
	awaitAssert.ThrowsAsync<JsonException>(()=>Serializer.DeserializeWrapper<NullArgTester_Mutable>(@"{""Point3DStruct"":null,""Int"":null,""ImmutableArray"":null}",options));