🐛 Fix poor merge behavior of Config + overrides

[TheAfroOfDoom]

while working on #1517, i realized that if you specified an empty object for feature: {}, it would effectively delete all other env.feature config properties.

example:

const base = {
	env: {
		feature: {
			a: true,
			b: true,
		}
	}
}

const override = {
	env: {
		feature: {},
	}
}

const ans = ConfigService.merge(base, override)

// ans => `{ env: { feature: {} } }`

this is undesired behavior for a config system -- users should not need to specify every adjacent property to override a nested config setting

---

turns out there was an old FIXME to do this anyway. we have a core.merge method that behaves the way we want so I just hooked into it in this PR