We tweaked the HTML and Arcade code that was provided via this forum and found that the following worked:
Fields to be displayed in the Pop-Up (NPL Status is the field with Null values):
Site ID: {SITE_ID}
PI Name: {PI_NAME}
Status: {STATUS}
NPL Status: {NPL_STATUS}
HTML Code for the Pop-Up Configuration:
<b>Site ID:</b> {SITE_ID}<br />
<b>PI Name:</b> {PI_NAME}<br />
<b>Status:</b> {STATUS}<br />
<span style="display:{expression/expr0}">
<b>NPL Status: </b>{NPL_STATUS}
</span>
Custom Attribute Expression in Arcade needed to remove both the field name and value when null :
IIF(isEmpty($feature.NPL_STATUS), "none", "inline")