Wait for server port to be written in a more robust manner

Previously, we just waited for the file to exist before trying to read
the port from it. This left a window where the file existed but its
contents had not be written which could result in a
NumberFormatException.

This commit now waits for the file to have a length that is greater
than zero.

See gh-7379

spring-boot-integration-tests/spring-boot-devtools-tests/src/test/java/org/springframework/boot/devtools/tests/DevToolsIntegrationTests.java

@@ -139,7 +139,7 @@ public class DevToolsIntegrationTests {
 
 	private int awaitServerPort() throws Exception {
 		long end = System.currentTimeMillis() + 20000;
-		while (!this.serverPortFile.exists()) {
+		while (this.serverPortFile.length() == 0) {
 			if (System.currentTimeMillis() > end) {
 				throw new IllegalStateException(
 						"server.port file was not written within 20 seconds");