Fix null dereference issues and improve error handling

[lupino3]

Address potential null dereference scenarios, remove deprecated URL constructor, and enhance error handling by returning empty strings for invalid inputs.

Add more unit tests for FPInstructionUtils.

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 6 lines in your changes missing coverage. Please review.

Project coverage is 52.53%. Comparing base (d03a42e) to head (34cb41d).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/main/java/org/edumips64/ui/swing/GUIHelp.java	0.00%	4 Missing ⚠️
...ava/org/edumips64/core/fpu/FPInstructionUtils.java	50.00%	1 Missing ⚠️
.../org/edumips64/ui/swing/MultiLineBasicTableUI.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1220      +/-   ##
============================================
+ Coverage     52.38%   52.53%   +0.15%     
- Complexity     1459     1468       +9     
============================================
  Files           252      252              
  Lines         10210    10214       +4     
  Branches       1102     1102              
============================================
+ Hits           5348     5366      +18     
+ Misses         4541     4525      -16     
- Partials        321      323       +2     

Files with missing lines	Coverage Δ
src/main/java/org/edumips64/core/IOManager.java	70.83% <100.00%> (ø)
src/main/java/org/edumips64/core/SymbolTable.java	49.05% <100.00%> (ø)
...rc/main/java/org/edumips64/core/parser/Parser.java	77.43% <ø> (ø)
...ava/org/edumips64/core/fpu/FPInstructionUtils.java	49.80% <50.00%> (+7.00%)	⬆️
.../org/edumips64/ui/swing/MultiLineBasicTableUI.java	0.00% <0.00%> (ø)
src/main/java/org/edumips64/ui/swing/GUIHelp.java	0.00% <0.00%> (ø)

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