Add get_section_information() and add_json_ld_metadata() functions

[greatislander]

\Pressbooks\Book::getBookInformation() returns an array of metadata for a book. The comparable method for retrieving an array of metadata for a section currently exists as a protected method in the section metadata API class. This PR:

• creates a function in the \Pressbooks\Metadata namespace to retrieve an array of section metadata
• creates a function in the same namespace to generate a JSON-LD metadata blob for Add JSON-LD book metadata to webbook pressbooks-book#434
• fixes an issue where a section with author metadata set to an empty string would not inherit the book author metadata

[codecov]

Codecov Report

Merging #1594 into dev will decrease coverage by <.01%.
The diff coverage is 85.71%.

@@             Coverage Diff             @@
##               dev    #1594      +/-   ##
===========================================
- Coverage     64.6%   64.59%   -0.01%     
+ Complexity    4550     4545       -5     
===========================================
  Files          121      121              
  Lines        19877    19883       +6     
===========================================
+ Hits         12842    12844       +2     
- Misses        7035     7039       +4

[dac514]

If the tests pass, then OK.