Fix bug with MI Parsing if there is a list with empty strings

A JavaScript string is a falsy value - but an empty string is valid in these positions - so need to explicitly check for undefined instead of just falsy.

src/backend/mi_parse.ts

@@ -213,7 +213,7 @@ export function parseMI(output: string): MINode {
 				let values = [];
 				values.push(value);
 				let remaining = output;
-				while (value = parseCommaValue())
+				while ((value = parseCommaValue()) !== undefined)
 					values.push(value);
 				output = output.substr(1); // ]
 				return values;

test/mi_parse.test.ts

@@ -173,4 +173,9 @@ suite("MI Parse", () => {
 		assert.equal(MINode.valueOf(obj[1], "@frame.fullname"), undefined);
 		assert.equal(MINode.valueOf(obj[1], "@frame.line"), undefined);
 	});
+	test("empty string values", () => {
+		let parsed = parseMI(`15^done,register-names=["r0","pc","","xpsr","","control"]`);
+		let result = parsed.result('register-names');
+		assert.deepEqual(result, ["r0", "pc", "", "xpsr", "", "control"]);
+	});
 });