Refactor ConvertTo Method in GenericListTypeConverter

[mamniashvili24]

This issue proposes a refactoring of the ConvertTo method in the GenericListTypeConverter class to improve code readability and maintainability.

Current behavior:
The original implementation iterates through the list and converts each element to a string using Convert.ToString with CultureInfo.InvariantCulture. However, it constructs the final string manually using a loop.
Proposed refactoring:
Leverage the Select method to create a sequence of culture-invariant strings for each element.
Utilize string.Join to efficiently concatenate these strings with a comma separator.
Benefits:
Enhances code clarity and conciseness.
Reduces redundancy and potential errors associated with manual string concatenation.
Aligns better with standard .NET practices for string manipulation.
Unit Test:
A unit test (CanConvertNullToString) has been added to ensure the converter handles null input gracefully, returning an empty string.

[skoshelev]

Closed #7198