helper/schema: Prevented panics during error to diagnostic conversion (#914)

Reference: hashicorp/terraform-plugin-sdk#789

Author: bflad

.changelog/914.txt

@@ -0,0 +1,7 @@
+```release-note:bug
+helper/schema: Prevented panics during `error` to diagnostic conversion for a non-`nil` error with an `Error()` method that returns an empty string (`""`)
+```
+
+```release-note:note
+helper/schema: Any returned non-`nil` `error` with an `Error()` method that returns an empty string (`""`), will now return an error diagnostic with an `"Empty Error String"` summary instead of a panic. Enabling Terraform logging at the `WARN` level (e.g. `TF_LOG=WARN terraform apply`) can help locate the problematic error by searching for the `detected empty error string` log message.
+```