IGNITE-5193: Hadoop: better handling of empty paths in environment variables. This closes apache#1928.

Author: Ivan Veselovskiy

modules/core/src/main/java/org/apache/ignite/internal/processors/hadoop/HadoopClasspathUtils.java

@@ -29,6 +29,7 @@
 import java.util.Collection;
 import java.util.LinkedList;
 import java.util.List;
+import org.apache.ignite.internal.util.typedef.F;
 
 /**
  * Hadoop classpath utilities.
@@ -258,7 +259,7 @@ private static String systemOrEnv(String name, String dflt) {
      * @return {@code True} if the given path denotes an existing directory.
      */
     public static boolean exists(String path) {
-        if (path == null)
+        if (F.isEmpty(path))
             return false;
 
         Path p = Paths.get(path);