avoid pushing undefined chip to chips list

[emps]

fix for #4193
undefined chip pushed into the items array while using md-on-append

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[emps]

I signed it!

[googlebot]

CLAs look good, thanks!

[topherfangio]

@emps Hey, sorry we haven't had a chance to respond yet! Was there a particular reason you closed this PR? Was it fixed by something else?

I am actively working on the chips but took a short break to try to finish a few things for 0.11, so I haven't had time to review this yet.