addrs: PartialExpandedModule.String correct for entirely-unexpanded

This logic was previously incorrect for any case where the number of
expanded steps was zero, causing the string representation to begin with
a dot when it ought not to.

internal/addrs/partial_expanded.go

@@ -155,7 +155,7 @@ func (pem PartialExpandedModule) String() string {
 		buf.WriteString(pem.expandedPrefix.String())
 	}
 	for i, callName := range pem.unexpandedSuffix {
-		if i > 0 || len(pem.unexpandedSuffix) != 0 {
+		if i > 0 || len(pem.expandedPrefix) != 0 {
 			buf.WriteByte('.')
 		}
 		buf.WriteString("module.")