add test for ExternalStylesFactory when styles element is missing, to bump coverage

src/file/styles/external-styles-factory.spec.ts

@@ -153,6 +153,12 @@ describe("External styles factory", () => {
             });
         });
 
+        it("should throw when style element isn't found", () => {
+            expect(() => new ExternalStylesFactory().newInstance(`<?xml version="1.0" encoding="UTF-8" standalone="yes"?><foo/>`)).to.throw(
+                "can not find styles element",
+            );
+        });
+
         it("should parse styles elements", () => {
             // tslint:disable-next-line:no-any
             const importedStyle = new ExternalStylesFactory().newInstance(externalStyles) as any;