To populate some dynamic text when the bodysummary
field is not available so you can use the computed field
for the bodysummary
field and in that computed field
you can check if any value is coming or not and on that basis, you can add some dynamic text.
You can check more about the computed field here https://doc.sitecore.com/xp/en/developers/90/platform-administration-and-architecture/create-a-computed-index-field.html