MDL-73826 phpunit: Allow curl mock responses to handle empty strings

Before this commit, is_empty() was being applied before returning
the mock response. But we want to be able to mock the empty response
for some tests, hence moving the condition to null/isset, that is
the value that array_pop() returns where there aren't more elements
in the array.

With that change performed, we can test lti_load_cartridge() with
empty responses, hence adding a new test for that.

mod/lti/tests/locallib_test.php

@@ -1945,6 +1945,20 @@ MwIDAQAB
         $this->assertEquals(16, $countwithproxyid); // 1 type, 15 proxies.
     }
 
+    /**
+     * Verify that empty curl responses lead to the proper moodle_exception, not to XML ValueError.
+     *
+     * @covers ::lti_load_cartridge()
+     */
+    public function test_empty_reponse_lti_load_cartridge() {
+        // Mock the curl response to empty string, this is hardly
+        // reproducible in real life (only Windows + GHA).
+        \curl::mock_response('');
+
+        $this->expectException(\moodle_exception::class);
+        lti_load_cartridge('http://example.com/mocked/empty/response', []);
+    }
+
     /**
      * Create an LTI Tool.
      *