before checking relations, check if relations exist inside model

[saadismail]

If some model (with readonly mixin enabled) does not have relations inside it, the mixin does not work. Here is the stack trace:

/project/node_modules/loopback-ds-readonly-mixin/lib/read-only.js:96
Object.keys(Model.definition.settings.relations).forEach(relationName => {
^

TypeError: Cannot convert undefined or null to object
at Function.keys ()
at EventEmitter.Model.on (/project/node_modules/loopback-ds-readonly-mixin/lib/read-only.js:96:12)
at EventEmitter.emit (events.js:193:13)
at EventEmitter.emit (domain.js:481:20)
at Function.app.model (/project/node_modules/loopback/lib/application.js:162:9)
at /project/node_modules/loopback-boot/lib/executor.js:203:9
at Array.forEach ()
at setupModels (/project/node_modules/loopback-boot/lib/executor.js:199:23)
at execute (/project/node_modules/loopback-boot/lib/executor.js:40:3)
at bootLoopBackApp (/project/node_modules/loopback-boot/index.js:154:3)
at Object. (/project/server/server.js:63:1)
at Module._compile (internal/modules/cjs/loader.js:816:30)
at Object.Module._extensions..js (internal/modules/cjs/loader.js:827:10)
at Module.load (internal/modules/cjs/loader.js:685:32)
at Function.Module._load (internal/modules/cjs/loader.js:620:12)
at Function.Module.runMain (internal/modules/cjs/loader.js:877:12)