Expand `EntityMenuItem` component test coverage (#30615)

* Do not render menu item that has both `action` and `link` props

* Add unit tests

* Address PR comment and refactor tests

frontend/src/metabase/components/EntityMenuItem/EntityMenuItem.tsx

@@ -32,7 +32,8 @@ const EntityMenuItem = ({
   onClose,
 }: EntityMenuItemProps): JSX.Element | null => {
   if (link && action) {
-    return <div />;
+    // You cannot specify both action and link props!
+    return null;
   }
 
   const content = (

frontend/src/metabase/components/EntityMenuItem/EntityMenuItem.unit.spec.js

@@ -41,5 +41,30 @@ describe("EntityMenuItem", () => {
         expect(screen.getByTestId("entity-menu-link")).toBeInTheDocument();
       });
     });
+
+    it("should not render if both action and link props are present", () => {
+      render(
+        <div data-testid="container">
+          <EntityMenuItem
+            title="A pencil icon"
+            icon="pencil"
+            link="/derp"
+            action={() => ({})}
+          />
+        </div>,
+      );
+
+      expect(screen.getByTestId("container")).toBeEmptyDOMElement();
+    });
+
+    it("should not render if neither action nor link props are present", () => {
+      render(
+        <div data-testid="container">
+          <EntityMenuItem title="A pencil icon" icon="pencil" />
+        </div>,
+      );
+
+      expect(screen.getByTestId("container")).toBeEmptyDOMElement();
+    });
   });
 });