Fixes bug #6838, quoteValueList fails with multiple empty values.

Link to bug:

https://github.com/zendframework/zf2/issues/6838

This may exist in other db platforms if the same do loop syntax is used. It looks like my whitespace (tab character) is different than yours? Anyway, the only code actually changed is that residing in the function quoteValueList, starting on line 157. 

Cheers.