fix: Random blank pages for some NGDIL credentials

[maiertech]

Description of change

Since the bug occurs during page rendering, I was unable to set a breakpoint and see what type of data is in a field when startsWith is invoked to check for a data URL. None of the credentialSubjects rendered with the DefaultRenderer contain anything other than string or something that can be coerced to a string.

Therefore, it is not clear why the DefaultRenderer should not be able to call startsWidth. To be on the safe side, it now checks if the type is string before checking if a field is a data URL.

Links to any relevant issues

• Fixes Credentials page is randomly blank #372.

How the change has been tested

Load the NGDIL credentials as described in #371 and test for every credential if it renders.

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