Is it safe to use XLR cable to connect single ended source with single ended amp?

That should work ok, Ghosthouse. To be completely certain, though, use your multimeter (I seem to recall that you have one) to make the following measurements before installing the cable and adapters in the system:

With the adapters attached to the cable, verify that there is continuity between the center pins of the two RCA connectors. Also verify that there is continuity between the ground sleeves of the two RCA connectors. Also verify that there is NO continuity between the center pin of either RCA connector and the corresponding ground sleeve.

Best regards,
-- Al
 

I would never do it.
First, Al is not 100% sure. And second, the sound is to be degraded.

Inna - I will take that under advisement; Understood: Sonically not ideal. I do have a multimeter and appreciate Al’s diagnostic recommendation. I can do that.  Thanks to both of you.