Reading a student grade when she has none yet results in an error.

[Tazaf]

When calling send_read_result() for a student that doesn't have any grade yet, some LMS (specifically Moodle) send back a XML response with a body value of "".

Here's an extract of the received XML sent back by a Moodle instance where the student did not have any grade for the activity :

<resultScore>
  <language>en</language>
  <textString/>
</resultScore>

As you can see, the value of textString is "".
This causes the lib to raise an error.

Having followed the chain of method, I've pinpointed the line that, according to me, causes the issue :

ims-lti/src/extensions/outcomes.coffee

Line 138 in 4df2936

score = parseFloat navigateXml(xml, 'imsx_POXBody.readResultResponse.result.resultScore.textString'), 10

The combination of parseFloat and navigateXml in the same line yield a score value of NaN (which is normal).

But the following check of isNan(score) returns an error as if this was an error case (which is not):

ims-lti/src/extensions/outcomes.coffee

Line 141 in 4df2936

callback new errors.OutcomeResponseError('Invalid score response'), false

I would expect a different behavior between a case where there is no grade yet and a case when the LMS sends back an actual invalid value.

I previously thought about returning a 0 value when there is no grade yet. But it might be confusing with the case where the student has received an actual 0 grade ? Dunno.