context keys: parser: report a parsing error on an empty string

Author: ulugbekna

src/vs/platform/contextkey/common/contextkey.ts

@@ -332,6 +332,7 @@ export class Parser {
 	parse(input: string): ContextKeyExpression | undefined {
 
 		if (input === '') {
+			this._parsingErrors.push('Expected an expression but got an empty string');
 			return undefined;
 		}