Update launch script to wait for pid to be written to pid file

Previously, the launch script would wait for the pid file to exist
before continuing. This didn’t work 100% of the time as it left a 
window where the file had been created but the PID had not been written
to it that could result in an incorrect report of the app failing to
start.

This commit updates the script to wait for the file to have a size
greater than zero before continuing. This ensures that the pid has
been written to the file before the pid is read from the file and used
to check that the process is running.

Closes gh-4923

spring-boot-tools/spring-boot-loader-tools/src/main/resources/org/springframework/boot/loader/tools/launch.script

@@ -85,7 +85,7 @@ isRunning() {
 await_file() {
   end=$(date +%s)
   let "end+=10"
-  while [[ ! -f "$1" ]]
+  while [[ ! -s "$1" ]]
   do
     now=$(date +%s)
     if [[ $now -ge $end ]]; then