See https://www.khronos.org/opengles/sdk/docs/man/xhtml/glDrawArrays.xml for example. It says the index type needs to be unsigned byte or unsigned short - it doesn't look like the indices parameter is optional. And actually the documentation for draw elements that you linked states the number of indices to draw, not vertices..
Uh, did you post the wrong link or did you mix something up? What you mean is glDrawElements, which, yes, needs indices. glDrawArrays like you (and MJP) have posted is the non-indexed rendering variant. If you look at the documentation you will see that dlDrawArrays doesn't have the "index type" parameter you describe, its in glDrawElements ;)