Placeholder preview image does not show up on exposure level page

[stscijgbot-jwql]

Issue JWQL-255 was created on JIRA by Bryan Hilbert:

The placeholder image, which says 'Preview image not available', is present when looking at the detector level page. But at the exposure level, when there is no preview image, the placeholder is not shown.

e.g. https://jwql.stsci.edu/NIRCam/exposure/jw06012101001_03106_00001/

In this case, there is no cal, psfalign, or psfmask preview images.