Fix NullPointerException in the "after the last transition" case

[dongockien92]

When the timezone belong to the "after the last transition" case (ex: "Asia/Bangkok"), we'll get the NullPointerException at FastISOTimestampFormatter

[codecov]

Codecov Report

Merging #789 (7986bd3) into main (eb692bd) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               main     #789   +/-   ##
=========================================
  Coverage     70.79%   70.80%           
  Complexity     1248     1248           
=========================================
  Files           167      167           
  Lines          4784     4788    +4     
  Branches        493      494    +1     
=========================================
+ Hits           3387     3390    +3     
  Misses         1151     1151           
- Partials        246      247    +1     

Impacted Files	Coverage Δ
...h/logback/composite/FastISOTimestampFormatter.java	95.45% <100.00%> (-0.98%)	⬇️

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[brenuart]

@dongockien92 Thanks for this report and your proposed PR.
I discovered other edge cases when reviewing your solution and decided to create another branch whose content is heavily inspired from your PR.
I'm closing your PR now without merging it - please follow #794 for more information.