MdSidenavlayout should not throw when no MdSidenav is present

[jeffbcross]

• Do you want to request a feature or report a bug?

Bug

• What is the current behavior?

If I have an MdSidenavLayout without an MdSidenav child, an exception will be raised. For instance, I wanted to avoid pre-compiling sidenav content in a pre-generated html document, so I threw an ngIf on the md-sidenav to hide it, which caused an error.

• Which version of Angular and Material, and which browser and OS does this issue affect?

Definitely alpha.1, and I think this is still present in alpha.2

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