Prevent counting on null

[SergioMendolia]

Salut! I got an error reported when parsing a Mobi file. This would prevent it

 "exception" => TypeError {
    #message: "count(): Argument #1 ($value) must be of type Countable|array, null given"
    #code: 0
    #file: "./vendor/kiwilan/php-ebook/src/Formats/Mobi/Parser/MobiParser.php"
    #line: 50
    trace: {
      ./vendor/kiwilan/php-ebook/src/Formats/Mobi/Parser/MobiParser.php:50 { …}
      ./vendor/kiwilan/php-ebook/src/Formats/Mobi/MobiModule.php:51 { …}
      ./vendor/kiwilan/php-ebook/src/Ebook.php:284 { …}
      ./vendor/kiwilan/php-ebook/src/Ebook.php:113 { …}

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.29%. Comparing base (b035de6) to head (32b0bb8).
Report is 3 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #100      +/-   ##
============================================
+ Coverage     90.19%   90.29%   +0.09%     
- Complexity      830      831       +1     
============================================
  Files            32       32              
  Lines          2550     2575      +25     
============================================
+ Hits           2300     2325      +25     
  Misses          250      250              

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[ewilan-riviere]

Thanks for this improvement!