Plist serialization: option to indent first dict element, expand empty elements

A rather annoying feature of ufonormalizer is that it

1. indents the first dict element in a plist, compared to what Apple writes out by default, though it is more consistent when viewed from an XML lens, and
2. expands empty elements.

Apple serializers, plist and Python's plistlib will write by default:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
	<key>lib</key>
	<dict>
		<key>org.unifiedfontobject.normalizer.imageReferences</key>
		<dict/>
	</dict>
</dict>
</plist>

but ufonormalizer will write:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
	<dict>
		<key>lib</key>
		<dict>
			<key>org.unifiedfontobject.normalizer.imageReferences</key>
			<dict>
			</dict>
		</dict>
	</dict>
</plist>

Note the <dict></dict> which ufonormalizer will write instead of just <dict/>.

Both are things that should maybe be writer options in plist, but I'm not sure if the author is open to this? We are, after all, doing semantically unnecessary things...

Start by locating ufonormalizer's plist serialization path and compare its output with Apple's serializers and Python's plistlib, as shown in the issue. Determine how writer options could control first-dictionary indentation and empty-element expansion; done means the requested output differences are configurable and covered by tests.