Omit empty mapping constructors from annotations in the style guide

[nipunayf]

Description

As specified in the spec https://ballerina.io/spec/lang/master/#section_9.1, the annotation value can be optional. Therefore, it is valid to write @test:Config for the following source code.

ballerina-dev-website/swan-lake/development-tutorials/source-code-dependencies/style-guide/annotations-documentation-and-comments.md

Lines 30 to 37 in d11afb0

	* If an annotation is empty, place it in a single line and
	do not have spaces between both braces.
	**Example,**
	```ballerina
	@test:Config {}
	```

IMO, omitting the mapping constructor when the value is absent should be considered best practice since it simplifies writing and improves readability.