refine use of "return" in database.php

* getRowInfo(): "else" is superfluous after "return"
* spotEntity(): idem
* getIPv4AddressNetworkId(): don't use "if" to reinvent ternary operator
* fetchIPv4AddressNetworkRow(): idem
* getIPv6AddressNetworkId(): idem
* fetchIPv6AddressNetworkRow(): idem
* searchByMgmtHostname(): idem
* objectTypeMayHaveParent(): idem
* loadScript(): idem
* getFileCache(): idem
* findFileByName(): idem
* acquireLDAPCache(): idem
* getUserIDByUsername(): idem
* getVLANSwitchInfo(): idem
* lookupEntityByString(): idem