[Bug]: Mocking function with optional parameter causing NullPointerException

[krishnadey30]

Description

I have encountered a NullPointerException when mocking a function that takes an optional parameter with a default value of () in Ballerina. When calling test:when(mockedFunction).withArguments() and passing () as one of the arguments, I get the following error:

error("java.lang.NullPointerException", message="Cannot invoke "Object.toString()" because the return value of "io.ballerina.runtime.api.values.BIterator.next()" is null")
    callableName: thenReturnFuncExt moduleName: ballerina.test.0 fileName: mock.bal lineNumber: 583
    callableName: thenReturn moduleName: ballerina.test.0.FunctionStub fileName: mock.bal lineNumber: 420
    callableName: testName moduleName: org.product.services$test.0.tests.lib_test fileName: tests/lib_test.bal lineNumber: 1116
    callableName: testName$lambda23$ moduleName: org.product.services$test.0.tests.test_execute-generated_2 fileName: tests/test_execute-generated_2.bal lineNumber: 27

Steps to Reproduce

function optionalJsonMock(json optionalJson = ()) returns string {
    return "This is optionalFunctionMock";
}

@test:Mock {functionName: "optionalJsonMock"}
test:MockFunction optionalJsonMockFn = new ();

@test:Config {}
function testOptionalJsonMock() {
    test:when(optionalJsonMockFn).withArguments(()).thenReturn("This is mocked return value");
    string response = optionalJsonMock();
    test:assertEquals(response, "This is mocked return value");
}

Affected Version(s)

2201.9.0

OS, DB, other environment details and versions

No response

Related area

-> Test Framework

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