fix: serialization null tag's value into LineProtocol (#283)

Author: bednar

CHANGELOG.md

@@ -1,5 +1,8 @@
 ## 4.1.0 [unreleased]
 
+### Bug Fixes
+1. [#283](https://github.com/influxdata/influxdb-client-java/pull/283): Serialization `null` tag's value into LineProtocol
+
 ## 4.0.0 [2021-11-26]
 
 ### Breaking Changes

client/src/main/java/com/influxdb/client/write/Point.java

@@ -347,7 +347,7 @@ private void appendTags(@Nonnull final StringBuilder sb, @Nullable final PointSe
             String key = tag.getKey();
             String value = tag.getValue();
 
-            if (key.isEmpty() || value.isEmpty()) {
+            if (key.isEmpty() || value == null || value.isEmpty()) {
                 continue;
             }
 

client/src/test/java/com/influxdb/client/write/PointTest.java

@@ -103,6 +103,17 @@ void tagEmptyValue() {
         Assertions.assertThat(point.toLineProtocol()).isEqualTo("h2o,location=europe level=2i");
     }
 
+    @Test
+    void tagNullValue() {
+
+        Point point = Point.measurement("h2o")
+                .addTag("location", "europe")
+                .addTag("log", null)
+                .addField("level", 2);
+
+        Assertions.assertThat(point.toLineProtocol()).isEqualTo("h2o,location=europe level=2i");
+    }
+
     @Test
     public void tagEscapingKeyAndValue() {