Fixed about dialog position on first view

[ham1]

Description

When viewing "About JMeter" dialog it was positioned with the top corner in the middle rather than in the centre of the screen because the first time the dialog wasn't pack()'ed so didn't have any size.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the code style of this project.
• I have updated the documentation accordingly.

[codecov-io]

Codecov Report

Merging #351 into trunk will increase coverage by <.01%.
The diff coverage is 0%.

@@             Coverage Diff              @@
##              trunk     #351      +/-   ##
============================================
+ Coverage     58.28%   58.29%   +<.01%     
- Complexity    10217    10219       +2     
============================================
  Files          1160     1160              
  Lines         74426    74427       +1     
  Branches       7343     7343              
============================================
+ Hits          43378    43384       +6     
+ Misses        28552    28549       -3     
+ Partials       2496     2494       -2

Impacted Files	Coverage Δ	Complexity Δ
...ore/org/apache/jmeter/gui/action/AboutCommand.java	0% <0%> (ø)	0 <0> (ø)	⬇️
...apache/jmeter/extractor/TestBoundaryExtractor.java	97.88% <0%> (ø)	17% <0%> (ø)	⬇️
...mpler/hc/JMeterPoolingClientConnectionManager.java	30.95% <0%> (+3.17%)	11% <0%> (+1%)	⬆️
...s/org/apache/jmeter/timers/PoissonRandomTimer.java	78.37% <0%> (+5.4%)	10% <0%> (+1%)	⬆️

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[ham1]

Can't see how this change caused a test failure...