Adds an additional check for enum value before converting it toLowerCase

Basically fixes an issue introduced with commit 2d298a3c9fca0a342251c665728cff04fc595a9b that causes conversion of undefined toLowerCase

TypeError: Cannot call method 'toLowerCase' of undefined
    at null.<anonymous> (/path/to/project/modules/elms/node_modules/sequelize/lib/dao.js:139:127)
    at EventEmitter.emit (events.js:95:17)
    at null.<anonymous> (/path/to/project/modules/elms/node_modules/sequelize/lib/dao-validator.js:45:17)
    at Hooks.runHooks (/path/to/project/modules/elms/node_modules/sequelize/lib/hooks.js:19:15)
    at null.<anonymous> (/path/to/project/modules/elms/node_modules/sequelize/lib/dao-validator.js:39:29)
    at Hooks.runHooks (/path/to/project/modules/elms/node_modules/sequelize/lib/hooks.js:19:15)
    at null.fct (/path/to/project/modules/elms/node_modules/sequelize/lib/dao-validator.js:26:27)
    at null.<anonymous> (/path/to/project/modules/elms/node_modules/sequelize/lib/emitters/custom-event-emitter.js:25:18)
    at processImmediate [as _immediateCallback] (timers.js:330:15)