Fix: exporting Markdown with missing authors doesn't crash anymore

[Mochitto]

Solves #10070 and maybe #10072

The previous version would crash due to author missing in the book's metadata.

-- line 38
local function prepareBookContent(book, formatting_options, highlight_formatting)
    [..........]
    content = content .. "##### " .. book.author:gsub("\n", ", ") .. "\n\n"
    [.........]

Added a conditional that sets the book author to a default (Author not found in book's metadata), so that the user can still have a feedback on missing metadata, while successfully exporting their notes.
The fix works both with export one file and export all files.

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This change is 

[Mochitto]

I haven't looked into it, but it might be worth to attribute default values to all of the properties of the entry/book objects so that they still get put in the documents and the user can fill in missing metadata.

I noticed some values are supposed to be ok even if they are missing, but making sure that all values are at least an empty string could avoid crashes depending on types (nil).
Or, if there could be expressive defaults for some things (for example for books metadata, such as authors), the user can have feedback on what metadata is missing.

Would be ok on taking on to myself (Assignees) adding the defaults if you find it a good idea.

[NiLuJe]

I tend to prefer explicit nil guards to empty string checks, but I can certainly see your point; RFC ;).

[Mochitto]

@NiLuJe What does RFC mean? :o I agree, explicit is better than implicit!

[NiLuJe]

Request For Comments ;).

[poire-z]

What does RFC mean?

just because it's today :) https://en.wikipedia.org/wiki/April_Fools%27_Day_Request_for_Comments