Bug - Markdown report throws error when all checks pass

[PeterDekkers]

Observed behaviour

When using the --ghmarkdown option to generate a Markdown report, and when all checks pass, an UnboundLocalError is thrown.

There is a previous issue which was accidentally closed: #5053

▶ fontbakery check-opentype fonts/otf/Fraunces9pt-Black.otf --ghmarkdown otf-opentype-checks.md  
Start ... running 53 individual check executions.

▓ [PSPPPPPSPPPPPPPPSSSSSSSPPPPPSPPPPPSPSPPSPPPSSSSSSSSSP] 100%
Total:
    ERROR: 0
    FATAL: 0
    FAIL: 0
    WARN: 0
    INFO: 0
    SKIP: 22
    PASS: 31



    Meaning of check results:

    An ERROR is something wrong with FontBakery itself, possibly a bug.
    A  FATAL is an extremely severe issue that must be addressed immediately.
    A  FAIL is a problem with the font that must be fixed.
    A  WARN is something that you should consider addressing.
    An INFO result simply prints something useful. Typically stats.
    A  PASS means the font looks good for the given checking routine.
And a  SKIP happens when the check does not apply to the given font.

    If you get ERRORs, please help us improve the tool by reporting them at
    https://github.com/fonttools/fontbakery/issues (but other kinds of bug
    reports and/or feature requests are also always welcome, of course!)




                           ,@.
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                   ,@@,.@@@.  @.@@@,.
                 ,@@. @@@.     @@. @@,.
         ,@@@.@,.@.              @.  @@@@,.@.@@,.
    ,@@.@.     @@.@@.            @,.    .@’ @’  @@,
  ,@@. @.          .@@.@@@.  @@’                  @,
 ,@.  @@.                                          @,
 @.     @,@@,.     ,                             .@@,
 @,.       .@,@@,.         .@@,.  ,       .@@,  @, @,
 @.                             .@. @ @@,.    ,      @
  @,.@@.     @,.      @@,.      @.           @,.    @’
   @@||@,.  @’@,.       @@,.  @@ @,.        @’@@,  @’
      \\@@@@’  @,.      @’@@@@’   @@,.   @@@’ //@@@’
       |||||||| @@,.  @@’ |||||||  |@@@|@||  ||
        \\\\\\\  ||@@@||  |||||||  |||||||  //
         |||||||  ||||||  ||||||   ||||||  ||
          \\\\\\  ||||||  ||||||  ||||||  //
           ||||||  |||||  |||||   |||||  ||
            \\\\\  |||||  |||||  |||||  //
             |||||  ||||  |||||  ||||  ||
              \\\\  ||||  ||||  ||||  //
               ||||||||||||||||||||||||

         No check is failing. Get a cupcake!
       <<Art by Tony de Marco, July 26, 2018>>

DONE!
Traceback (most recent call last):
  File "/Users/stephennixon/type-repos/00-client-type-repos/Fraunces/venv/bin/fontbakery", line 8, in <module>
    sys.exit(main())
             ^^^^^^
  File "/Users/stephennixon/type-repos/00-client-type-repos/Fraunces/venv/lib/python3.11/site-packages/fontbakery/cli.py", line 520, in main
    reporter.write()
  File "/Users/stephennixon/type-repos/00-client-type-repos/Fraunces/venv/lib/python3.11/site-packages/fontbakery/reporters/serialize.py", line 57, in write
    fh.write(self.template(self.getdoc()))
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/stephennixon/type-repos/00-client-type-repos/Fraunces/venv/lib/python3.11/site-packages/fontbakery/reporters/ghmarkdown.py", line 119, in template
    total=num_checks,
          ^^^^^^^^^^
UnboundLocalError: cannot access local variable 'deprecation_warning' where it is not associated with a value

Expected behaviour

No error should be thrown.

Resources and steps needed to reproduce

See #5053