Fixed Flash\Session::output() when session is empty

[zsilbi]

Hello!

• Type: bug fix
• Link to issue: [BUG] Flash\Session output() throws exception when session is empty #14023

In raising this pull request, I confirm the following:

• I have read and understood the Contributing Guidelines
• I have checked that another pull request for this purpose does not exist
• I wrote some tests for this PR
• I updated the CHANGELOG

Small description of change:

Fixed Phalcon\Flash\Session::output() not to throw an exception when there are no messages stored in session.

Thanks,
zsilbi

[codecov]

Codecov Report

Merging #14024 into 4.0.x will increase coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##            4.0.x   #14024      +/-   ##
==========================================
+ Coverage   68.41%    68.5%   +0.08%     
==========================================
  Files         470      470              
  Lines       95150    95153       +3     
==========================================
+ Hits        65100    65183      +83     
+ Misses      30050    29970      -80

Impacted Files	Coverage Δ
ext/phalcon/flash.zep.c	87.36% <0%> (+2.19%)	⬆️
ext/phalcon/flash/session.zep.c	42.39% <0%> (+40.73%)	⬆️

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[niden]

Thank you @zsilbi