Add @Nullable annotations

[Shadow-Devil]

Purpose

Adds @nullable annotations where null values are passed as arguments or assigned to variables.

Fixes #43386

Approach

Let IntelliJ generate the @nullable annotations based on its static analysis

Samples

Provide high-level details about the samples related to this feature.

Remarks

List any other known issues, related PRs, TODO items, or any other notes related to the PR.

Check List

• Read the Contributing Guide
• Updated Change Log
• Checked Tooling Support (#)
• Added necessary tests
  • Unit Tests
  • Spec Conformance Tests
  • Integration Tests
  • Ballerina By Example Tests
• Increased Test Coverage
• Added necessary documentation
  • API documentation
  • Module documentation in Module.md files
  • Ballerina By Examples

[gimantha]

@Shadow-Devil Shall we check on the build failures in github actions?

[Shadow-Devil]

Some libraries fail because of Spotbugs errors. They are using methods that are now annotated with @Nullable and they dont check it.

[codecov]

Codecov Report

Attention: Patch coverage is 70.37037% with 48 lines in your changes missing coverage. Please review.

Project coverage is 77.39%. Comparing base (0c56743) to head (abce447).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
...rc/main/java/io/ballerina/projects/BalaWriter.java	12.50%	6 Missing and 1 partial ⚠️
.../java/org/ballerinalang/toml/model/Dependency.java	25.00%	6 Missing ⚠️
...allerina/compiler/api/impl/ExpectedTypeFinder.java	20.00%	0 Missing and 4 partials ⚠️
...rc/main/java/io/ballerina/cli/cmd/CommandUtil.java	66.66%	3 Missing ⚠️
...internal/repositories/RemotePackageRepository.java	0.00%	3 Missing ⚠️
...o/ballerina/cli/task/CreateTestExecutableTask.java	0.00%	1 Missing and 1 partial ⚠️
...rc/main/java/io/ballerina/cli/utils/TestUtils.java	0.00%	1 Missing and 1 partial ⚠️
...l/type/builders/BallerinaSingletonTypeBuilder.java	0.00%	1 Missing and 1 partial ⚠️
...main/java/io/ballerina/projects/ModuleContext.java	66.66%	1 Missing and 1 partial ⚠️
...o/ballerina/projects/internal/BlendedManifest.java	60.00%	0 Missing and 2 partials ⚠️
... and 12 more

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #43393      +/-   ##
============================================
- Coverage     77.40%   77.39%   -0.02%     
+ Complexity    58600    58593       -7     
============================================
  Files          3446     3446              
  Lines        219114   219123       +9     
  Branches      28965    28955      -10     
============================================
- Hits         169604   169582      -22     
- Misses        40080    40118      +38     
+ Partials       9430     9423       -7     

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[Shadow-Devil]

@gimantha required pipeline went through but some std libs will require some adjustments since they use methods that are now annotated with @Nullable so spotbugs complaints