Allow only non null values to flow to influxDB

InfluxDB does not allow null values for tags or fields. When null is propagated to influx, it throws an exception and the data is lost. To avoid this populate the Builder with only non null values.

Author: maanasa

src/main/java/org/influxdb/dto/Point.java

@@ -276,9 +276,13 @@ private void addFieldByAttribute(final Object pojo, final Field field, final Col
         }
 
         if (column.tag()) {
-          this.tags.put(fieldName, (String) fieldValue);
+          if(fieldValue != null) {
+            this.tags.put(fieldName, (String) fieldValue);
+          }
         } else {
-          this.fields.put(fieldName, fieldValue);
+          if(fieldValue != null) {
+            this.fields.put(fieldName, fieldValue);
+          }
         }
 
       } catch (IllegalArgumentException | IllegalAccessException e) {