Function defined

defined<T>(val): val is Exclude<T, undefined | null>
from https://github.com/CesiumGS/cesium/blob/master/Source/Core/defined.js
Type Parameters
- T
Parameters
- val: T
  The object.
Returns val is Exclude<T, undefined | null>
Returns true if the object is defined, returns false otherwise.
Example
```
if (defined(positions)) {
  doSomething();
} else {
  doSomethingElse();
}
```
- Defined in defined.ts:16