Add support for parameterless mock() invocations

[fm2003]

A student wrote this:

class SecurityScannerTest {
    private SecurityOfficer securityOfficer;
    // ...

    private SecurityScanner scanner;

    @BeforeEach
    public void setUp() {
        securityOfficer = mock();
        // ...
    }
}

This is now valid Mockito syntax, so we want to make sure that we detect such a way to create a mock of a class (in this case the SecurityOfficer class).