Allow not providing values to selected configurable variables through CLI

[ravinperera00]

Purpose

A bug in the CLI does not allow users to leave out values (by not providing values) for configurable variables that have an unsupported type and causes an error. This PR fixes that issue.
Additionally, the CLI does not support accepting values for configurable variables with a union type that has null as a member type (ex: int?). This PR adds that support.

Fixes #42859

Approach

Bug fix

When considering unsupported configurable values in the CLI, there could be two cases:

Case 1
A type that is not supported through the CLI is used as a configurable variable but the value is not provided through the CLI. This should not be a problem and the CLI should allow leaving the value for that out as long as a default values is provided through other methods.

Case 2
A type that is not supported through the CLI is used as a configurable variable but the value for that is provided through the CLI. This should cause and error stopping the program.

Checking whether the variable value is null before executing the checks regarding the type can produce the expected outcome.

Improvement

The Ballerina spec mentions the following regarding the supported types for configurable values.

Configurable values can be supplied with the built-in command-line option -C,

-Ckey=value

Key syntax:
key:= [[org .] module .] variable

Define S as:
type S boolean | int | float | decimal | string | xml

In the first phase, only the configurable variables with type S? are supported. We can support anydata[], map<anydata> and table<map<anydata>> later as needed.

Therefore types such as int? should be supported through the CLI. This can be achieved by accepting the () type as a valid member type within union types.

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

[HindujaB]

Let's shorten the PR title as well. We can elaborate on it inside the description and approach.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.34%. Comparing base (1a84008) to head (8380e37).
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master   #42978   +/-   ##
=========================================
  Coverage     77.34%   77.34%           
- Complexity    58233    58236    +3     
=========================================
  Files          3450     3450           
  Lines        218919   218920    +1     
  Branches      28726    28726           
=========================================
+ Hits         169315   169316    +1     
  Misses        40281    40281           
  Partials       9323     9323           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.