[FrameworkBundle] Ignore missing directories in about command

[ro0NL]

Q	A
Branch?	5.4
Bug fix?	yes IMHO
New feature?	no
Deprecations?	no
Tickets	Fix #...
License	MIT
Doc PR	symfony/symfony-docs#...

This fixes our CI when var/log is missing.

Before:

In AboutCommand.php line 105:
                                                                                                              
  [UnexpectedValueException]                                                                                  
  RecursiveDirectoryIterator::__construct(/app/var/log): Failed to open directory: No such file or directory  

After:

  Cache directory      ./var/cache/prod (939 KiB)  
  Build directory      ./var/cache/prod (939 KiB)  
  Log directory        ./var/log (n/a)   

[ro0NL]

the abstraction for "formatted file size" is any string, hence this looks appropriate to me

[fabpot]

Thank you @ro0NL.