fix: .clear method generates namespace even if key is passed

[Mdaadm]

Fixes #451

calling

FastAPICache.clear(key="value")

method generates

namespace = cls._prefix + (":" + namespace if namespace else "")

even if key is passed. It then goes to

cls._backend.clear(namespace, key)

where key is never used because of this if-else statement:

if namespace:
    lua = f"for i, name in ipairs(redis.call('KEYS', '{namespace}:*')) do redis.call('DEL', name); end"
    return await self.redis.eval(lua, numkeys=0)  # type: ignore[union-attr,no-any-return]
elif key:
    return await self.redis.delete(key)  # type: ignore[union-attr]
return 0

Same for inmemory backend realisation.

With proposed changes clearing cache by key will work correctly, without clearing whole namespace