* Prevent IndexError when deleting multiple lines
The old code will raise `IndexError: list assignment index out of range` when deleting multiple lines because the indexes of the original and the copy get out of sync. Solved by deleting from the high indexes first so the lower ones remain stable.
* Don't load configuration if nothing to load
Instead of sending an empty candidate config (for example because the candidate only consisted of `delete` lines, and all of them were filtered out by `filter_delete_statements`) just return. JunOS seems to get confused by empty changes, and if the candidate config is empty then it's a no-op anyway.
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