Removed Some Redundant Code

[martindevans]

Removed some minor redundant code, as highlighted by R#

• Mostly null handling code that's unnecessary because the type statically cannot be null
• Null suppression of values already known to be not null
• Sealed overrides when the class is sealed

Name and Content properties of CustomSection are documented as allowing null to be assigned. I haven't changed that behaviour, since it's specifically documented I assume it's important! Adding AllowNull lets null be assigned, but does not allow null to be returned.