[SR-16072] Stop using a "placeholder abstract" in DocumentationNode

[QuietMisdreavus]

Previous ID	SR-16072
Radar	rdar://90982942
Original Reporter	@QuietMisdreavus
Type	Improvement

Additional Detail from JIRA

Votes	0
Component/s	Swift-DocC
Labels	Improvement
Assignee	None
Priority	Medium

md5: 8c315d981b6d5e65d33a82941834812a

Issue Description:

When a DocumentationNode is first initialized, it gets a basic set of data loaded in. Some of the data is left blank and loaded in from symbol data later on. In the initializer, the abstractSectionVariants gets populated with a "Placeholder Abstract". This placeholder is unnecessary; it will get overwritten by whatever the symbol information has, even if it's blank. Swift-DocC and Swift-DocC-Render can handle blank abstracts, so this placeholder should instead just be .empty to avoid the useless allocation.

[QuietMisdreavus]

This can be a good way for a new user to get used to the Swift-DocC contribution flow! The code that needs to be changed are these lines in DocumentationNode.swift - the creation of the DocumentationDataVariants there should instead be a call to .empty.

[QuietMisdreavus]

Closed by #113