Partial fix for #7: undefined property error

The first error has a separate cause from the rest of the warnings quoted in #7:

`Notice: Undefined property: stdClass::$text in /Users/brsmith/Moodle/moodle-2.7/moodle/blocks/learning_plan/block_learning_plan.php on line 51`

The `text` property is appended to without first initializing it to an empty string.