Helper function for Serde serialization, allowing us to skip serialization
of default config values. Rationale: if we don’t skip serialization of
defaults, if someone serializes a config with some default values, they’re
“pinning” the current defaults as their choices for all time, and we have no
way to distinguish between fields they configured explicitly and ones they
passed through from the defaults. If we skip serializing default values,
then we know every value in the config was explicitly set.