Fix null point exception when ssl_truststore_location is nil (#256)

* Fix null point exception when ssl_truststore_location is nil

Author: kunna-ujet

lib/logstash/inputs/kafka.rb

@@ -332,7 +332,7 @@ def create_consumer(client_id)
 
   def set_trustore_keystore_config(props)
     props.put("ssl.truststore.type", ssl_truststore_type) unless ssl_truststore_type.nil?
-    props.put("ssl.truststore.location", ssl_truststore_location)
+    props.put("ssl.truststore.location", ssl_truststore_location) unless ssl_truststore_location.nil?
     props.put("ssl.truststore.password", ssl_truststore_password.value) unless ssl_truststore_password.nil?
 
     # Client auth stuff