tslint: Enable no-null-keyword

I find this rule quite useful so that we use undefined all the time.
The rationale on the rule's documentation page is quite clear:

https://palantir.github.io/tslint/rules/no-null-keyword/

src/backend/mi2/mi2.ts

@@ -544,7 +544,7 @@ export class MI2 extends EventEmitter implements IBackend {
 								this.breakpoints.set(newBrk, bkptNum);
 								resolve([true, newBrk]);
 							} else {
-								resolve([false, null]);
+								resolve([false, undefined]);
 							}
 						}, reject);
 					}

src/mibase.ts

@@ -294,7 +294,7 @@ export class MI2DebugSession extends DebugSession {
 		this.miDebugger.getStack(args.levels, args.threadId).then(stack => {
 			const ret: StackFrame[] = [];
 			stack.forEach(element => {
-				let source = null;
+				let source = undefined;
 				let file = element.file;
 				if (file) {
 					if (this.isSSH) {

tslint.json

@@ -2,6 +2,7 @@
 	"defaultSeverity": "error",
 	"rules": {
 		"no-var-keyword": true,
-		"prefer-const": true
+		"prefer-const": true,
+		"no-null-keyword": true
 	}
 }