WIP: fix null values in lists (grpc)

...ain/java/org/caosdb/server/accessControl/UserSources.java

...database/backend/implementation/MySQL/MySQLListRoles.java

...database/backend/implementation/MySQL/MySQLListUsers.java

...abase/backend/implementation/MySQL/MySQLLogUserVisit.java

.../backend/implementation/MySQL/MySQLRetrieveEntityACL.java

...sdb/server/database/backend/interfaces/ListRolesImpl.java

...sdb/server/database/backend/interfaces/ListUsersImpl.java

...er/database/backend/interfaces/RetrieveEntityACLImpl.java

...caosdb/server/database/backend/transaction/ListRoles.java

...caosdb/server/database/backend/transaction/ListUsers.java

...ase/backend/transaction/RetrieveEntityACLTransaction.java

.../server/database/backend/transaction/UpdateUserRoles.java

...aosdb/server/grpc/AccessControlManagementServiceImpl.java

src/main/java/org/caosdb/server/grpc/AuthInterceptor.java

...n/java/org/caosdb/server/grpc/CaosDBToGrpcConverters.java

.../org/caosdb/server/grpc/EntityTransactionServiceImpl.java

...n/java/org/caosdb/server/grpc/GrpcToCaosDBConverters.java

...a/org/caosdb/server/transaction/ListRolesTransaction.java

...a/org/caosdb/server/transaction/ListUsersTransaction.java

src/main/java/org/caosdb/server/transaction/RetrieveACL.java