get_response_jwt crashes when deep_link_settings['data'] is not set

[etsikt]

Steps to reproduce:

1. Issue a deep linking request message without specifying deep_link_settings['data'].
2. Respond using output_response_form function in LTI_Deep_Link.php
3. Notice that the code crashes because deep_link_settings['data'] is not set:
   [2021-11-02 15:26:50] production.ERROR: Undefined index: data {"exception":"[object] (ErrorException(code: 0): Undefined index: data at /var/www/vendor/imsglobal/lti-1p3-tool/src/lti/LTI_Deep_Link.php:28)
   Link to GitHUB: https://github.com/IMSGlobal/lti-1-3-php-library/blob/3a192de99f3783d76caea462b0d04db28569c123/src/lti/LTI_Deep_Link.php#L28

Expected behaviour:
According to https://www.imsglobal.org/spec/lti-dl/v2p0#deep-linking-settings data is not
a required value, and the code should therefore check whether it is set or not and handle both situations.

[sunny047]

Hi,

Any update on this issue? I am testing this library in a laravel and facing this issue while using the output_response_form function.

[etsikt]

I ended up doing a fork with these changes: matematikk-mooc@eb6a4fb