No account existence check on low-level call
executeMetaTransaction uses a low-level call without checking that the destination address actually has code, so calls to non-existent accounts silently return success.
Description
Low-level calls call/delegatecall/staticcall return true even if the account called is non-existent (per EVM design). Account existence must be checked prior to calling.
Impact
The function executeMetaTransaction uses call (which returns boolean), however there is no account existence check for the destination address to.
If it doesn't exist, for some reason, call will still return true (not throw an exception) and successfully pass the return value check on the next line.
Considering the fact that this function is external and can be executed by anyone, there is risk of exploit or misuse.
For reference, see this related high-risk severity finding from Trail of Bit's audit of Hermez Network.
Recommendation
Check for account-existence before the call() to make this safely extendable to user-controlled address contexts in future.
