Prevent "undefined index" notice if $parts array is empty in Index\Index#removeIndexedDefinition

[asccc]

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If $level reaches 0 with only one more element in the $parts array, the next call to array_slice($parts, 0, $level) will return an empty array. This will trigger a "undefined index" notice in the next recursion on $part = $parts[$level]. This notice gets promoted to an ErrorException in the error-handler (if the local php.ini is configured to report notices) which will cancel further processing.

Fixes #695