Return 0 from MemcachedCache._increment when the result is 0

[uttam12331]

Problem

MemcachedCache.increment reports the wrong value when a counter is decremented to exactly 0:

await cache.set("counter", 5)
await cache.increment("counter", -5)   # returns -5, but the stored value is 0

aiomcache's incr/decr return the new value, or None if the key is missing. _increment ends with:

return incremented or delta

The or delta fallback is there to handle the missing-key case (incremented is None). But 0 is also falsy, so when the counter reaches exactly 0, 0 or delta returns delta instead of 0.

Impact

A decrement (or increment) whose true result is 0 returns the delta instead — increment("counter", -5) on a counter of 5 returns -5 while a subsequent get returns 0. The memory and valkey backends both return the true post-increment value (and BaseCache.increment documents the return as "Value of the key once incremented"), so only the memcached backend diverges.

Fix

return incremented if incremented is not None else delta

This keeps the missing-key behavior (return delta when the client returned None) while correctly returning a real 0.

Tests

Added test_increment_result_zero to tests/ut/backends/test_memcached.py, asserting that when the client's decr returns 0, _increment returns 0. It fails on master (returns -5) and passes with the fix.