configs/configupgrade: fix panic on nil hilNode (#22181)

In some cases (see #22020 for a specific example), the parsed hilNode
can be nil. This causes a series of panics. Instead, return an error and
move on.

configs/configupgrade/upgrade_expr.go

@@ -110,18 +110,22 @@ Value:
 						Subject:  hcl1PosRange(filename, tv.Pos).Ptr(),
 					})
 				}
+				if hilNode != nil {
 					if _, ok := hilNode.(*hilast.Output); !ok {
 						// hil.Parse usually produces an output, but it can sometimes
 						// produce an isolated expression if the input is entirely
 						// a single interpolation.
+						if hilNode != nil {
 							hilNode = &hilast.Output{
 								Exprs: []hilast.Node{hilNode},
 								Posx:  hilNode.Pos(),
 							}
 						}
+					}
 					interpDiags := upgradeHeredocBody(&buf, hilNode.(*hilast.Output), filename, an)
 					diags = diags.Append(interpDiags)
 				}
+			}
 			if !strings.HasSuffix(body, "\n") {
 				// The versions of HCL1 vendored into Terraform <=0.11
 				// incorrectly allowed the end marker to appear at the end of