cost and currency properties can be undefined

[ewallah]

There is no check if cost and currency are set.

As a result some reports built with reportbuilder and the core_course PHPUNIT tests fail:

participants_test (core_course\reportbuilder\datasource\participants_test)
✔ Datasource default [411.50 ms]
✘ Datasource non default columns [510.15 ms]
│
│ Undefined property: stdClass::$cost
│
│ enrol/wallet/lib.php:193
│ enrol/classes/reportbuilder/local/entities/enrol.php:122
│ reportbuilder/classes/local/report/column.php:698
│ reportbuilder/classes/table/custom_report_table.php:209
│ reportbuilder/tests/helpers.php:55
│ course/tests/reportbuilder/datasource/participants_test.php:199
│ lib/phpunit/classes/advanced_testcase.php:76
│
✔ Course last access aggregation [306.33 ms]
✔ Completion days taking course aggregation [470.45 ms]
✔ Datasource filters with data set #0 [422.17 ms]
✔ Datasource filters with data set #1 [416.77 ms]
✔ Datasource filters with data set #2 [409.21 ms]
✔ Datasource filters with data set #3 [408.47 ms]
✔ Datasource filters with data set #4 [422.21 ms]
✔ Datasource filters with data set #5 [426.89 ms]
✔ Datasource filters with data set #6 [417.81 ms]
✔ Datasource filters with data set #7 [425.02 ms]
✔ Datasource filters with data set #8 [442.55 ms]
✔ Datasource filters with data set #9 [434.12 ms]
✔ Datasource filters with data set #10 [435.77 ms]
✔ Datasource filters with data set #11 [410.45 ms]
✔ Datasource filters with data set #12 [423.98 ms]
✔ Datasource filters with data set #13 [430.42 ms]
✔ Datasource filters with data set #14 [413.47 ms]
✔ Datasource filters with data set #15 [425.26 ms]
↩ Stress datasource [68.00 ms]
│
│ PHPUNIT_LONGTEST is not defined
│
│ course/tests/reportbuilder/datasource/participants_test.php:522
│ lib/phpunit/classes/advanced_testcase.php:76
│