Fix crash in SBStructuredData::GetDescription() when there's no StructuredDataPlugin.

Also, use the StructuredData::Dump method to print the StructuredData if there
is no plugin, rather than just returning an error.

Differential Revision: https://reviews.llvm.org/D88266

lldb/test/API/python_api/sbstructureddata/TestStructuredDataAPI.py

@@ -35,6 +35,13 @@ class TestStructuredDataAPI(TestBase):
         # Tests for invalid data type
         self.invalid_struct_test(example)
 
+        # Test that GetDescription works:
+        s.Clear()
+        error = example.GetDescription(s)
+        self.assertTrue(error.Success(), "GetDescription works")
+        if not "key_float" in s.GetData():
+            self.fail("FAILED: could not find key_float in description output")
+        
         dict_struct = lldb.SBStructuredData()
         dict_struct = example.GetValueForKey("key_dict")