Code cleanup and minor fixes

[NewbieOrange]

Removed duplicated null checking, fix several potential NullPointerException.

Also fixed NPE when user Ctrl+D in password prompt.

[remkop]

Thank you!
I took a quick look; I will take a more detailed look later.

[remkop]

Overall some good catches, but there are some places where I like the current style.
Specifically:

• I generally think String concatenation with + gives more readable code than StringBuilder.append
• Using Map.Entry to loop over maps is more efficient, but tends to give longer and less readable code.

[codecov-io]

Codecov Report

Merging #1040 into master will decrease coverage by 0.01%.
The diff coverage is 82.97%.

@@             Coverage Diff              @@
##             master    #1040      +/-   ##
============================================
- Coverage     94.37%   94.35%   -0.02%     
  Complexity      443      443              
============================================
  Files             2        2              
  Lines          6474     6469       -5     
  Branches       1728     1726       -2     
============================================
- Hits           6110     6104       -6     
- Misses           96       98       +2     
+ Partials        268      267       -1     

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/picocli/CommandLine.java	94.19% <82.97%> (-0.03%)	304.00 <0.00> (ø)

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[NewbieOrange]

Reverted those changes.

[remkop]

Looks good, thank you!

[remkop]

Merged into master.
Thank you for the pull request!