[ZEPPELIN-2152] Fix for npe in Helium loading when no proxies are set

Emergency fix for npe when calling Helium and with no proxies env variables set.

Bug Fix

https://issues.apache.org/jira/browse/ZEPPELIN-2152

* clone new zeppelin repository
* unset http_proxy/https_proxy env variables (if set)
* build ` mvn clean package -DskipTests;`
* start zeppelin instance

* Does the licenses files need update? No
* Is there breaking changes for older versions? No
* Does this needs documentation? No

Author: Nelson Costa <nelson.costa85@gmail.com>

Closes #2380 from necosta/zeppelin2152-patch and squashes the following commits:

cb25cd7 [Nelson Costa] [ZEPPELIN-2152] Fix for npe in Helium loading when no proxies are set

(cherry picked from commit 88c5c3c)
Signed-off-by: Lee moon soo <moon@apache.org>

zeppelin-zengine/src/main/java/org/apache/zeppelin/helium/HeliumVisualizationFactory.java

@@ -122,9 +122,10 @@ private ProxyConfig getProxyConfig(boolean isSecure) {
         System.getenv("HTTPS_PROXY") : System.getenv("https_proxy");
 
     try {
-      if (isSecure)
+      if (isSecure && StringUtils.isNotBlank(httpsProxy))
         proxies.add(generateProxy("secure", new URI(httpsProxy)));
-      else proxies.add(generateProxy("insecure", new URI(httpProxy)));
+      else if (!isSecure && StringUtils.isNotBlank(httpsProxy))
+        proxies.add(generateProxy("insecure", new URI(httpProxy)));
     } catch (Exception ex) {
       logger.error(ex.getMessage(), ex);
     }