Clear oneofs correctly

[rodaine]

When protogofakeit attempts to fake out a oneof union, depending on the random value it rolls it may choose to have the oneof have no set fields. Instead of ignoring it, the faker attempts to clear the oneof, in the event the message already had fields set within the oneof. The previous logic handled this by populating the first field in the oneof with its default (protoreflect.FieldDescriptor#Default), and then clearing that field, which would force the oneof to be empty. This worked fine for scalar fields, but a message field's default is nil. Calling protoreflect.Message#Set with a nil value results in a panic.

Two solutions were available here. First, is to instead do protoreflect.Message#NewField which would return a viable message value that could be set and subsequently cleared, but I realized that this was unnecessary work to clear the field. Instead, the message is now interrogated about which field in the oneof is set (if any) and clear that specific field. Much better imo.

Fixes #5