Fix: chaining of GRADLE_OPTS (#11878)

Added test to avoid inserting and empty space in GRADLE_OPTS, and moved echo command outside of if to make it effective

Author: andsel

ci/unit_tests.bat

@@ -41,9 +41,13 @@ echo Using drive !use_drive! for %WORKSPACE%
 
 echo Running core tests..
 if defined BUILD_JAVA_HOME (
-  set GRADLE_OPTS="%GRADLE_OPTS% -Dorg.gradle.java.home=%BUILD_JAVA_HOME%"
-  echo GRADLE_OPTS is: %GRADLE_OPTS%, BUILD_JAVA_HOME: %BUILD_JAVA_HOME%
+  if defined GRADLE_OPTS (
+    set GRADLE_OPTS=%GRADLE_OPTS% -Dorg.gradle.java.home=%BUILD_JAVA_HOME%
+  ) else (
+    set GRADLE_OPTS=-Dorg.gradle.java.home=%BUILD_JAVA_HOME%
+  )
 )
+echo Invoking Gradle, GRADLE_OPTS: %GRADLE_OPTS%, BUILD_JAVA_HOME: %BUILD_JAVA_HOME%
 call .\gradlew.bat test --console=plain --no-daemon --info
 
 if errorlevel 1 (