Fix a bug in _valueForItem

[jydelort-at-google-com]

dashToCamelCase fails with indexOf called on null if this.attrForSelected is null.

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[valdrinkoshi]

Closing this in favor of #163

[bicknellr]

Hi, @jydelort-at-google-com, could you add a test for this situation?

[bicknellr]

(going back to #163)