Only throw PortInUseException if port is set

Refine the `PortInUseException` logic in `NettyWebServer` to only throw
an exception if the port is set. The prevents a misleading exception
from being thrown when a domain socket is being used.

Closes gh-24529

Author: philwebb

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/embedded/netty/NettyWebServer.java

@@ -101,7 +101,7 @@ public void start() throws WebServerException {
 			}
 			catch (Exception ex) {
 				PortInUseException.ifCausedBy(ex, ChannelBindException.class, (bindException) -> {
-					if (!isPermissionDenied(bindException.getCause())) {
+					if (bindException.localPort() > 0 && !isPermissionDenied(bindException.getCause())) {
 						throw new PortInUseException(bindException.localPort(), ex);
 					}
 				});