@Test
    public void testGetExtensionLoaderIsNull() {
        try {
            ExtensionLoader.getExtensionLoader(null);
            fail();
        } catch (NullPointerException expected) {
            assertThat(expected.getMessage(), containsString("extension clazz is null"));
        }
    }
test ExtensionLoader.getExtensionLoader() null param case.
shenyu-spi/src/test/java/org/apache/shenyu/spi/ExtensionLoaderTest.java