From the patch notes of the most recent GMS update (2.3.7.606):

• Function calls can now be made with missing arguments, these are then passed as undefined and either optional arguments or the called function can handle that
• I.e., you can now write MyFunctionCall(1,2,,,5) and get the same result as if you had written MyFunctionCall(1, 2, undefined, undefined, 5)
  

This feature is causing a compile time error to be thrown:
// Error in globalScript at @[<script>:<line_number>]:
// unexpected symbol "," in expression