Fix YamlConfiguration dumping null values and empty tree nodes

Core/src/main/java/com/songoda/core/configuration/yaml/YamlConfiguration.java

@@ -209,6 +209,8 @@ public class YamlConfiguration implements IConfiguration, HeaderCommentable, Nod
         String headerCommentLines = generateHeaderCommentLines();
         writer.write(headerCommentLines);
 
+        cleanValuesMap(this.values);
+
         if (this.values.size() > 0) {
             if (headerCommentLines.length() > 0) {
                 writer.write(this.yamlDumperOptions.getLineBreak().getString());
@@ -343,4 +345,23 @@ public class YamlConfiguration implements IConfiguration, HeaderCommentable, Nod
             ++currentKeyIndex;
         }
     }
+
+    /**
+     * This takes a map and removes all keys that have a value of null.<br>
+     * Additionally, if the value is a {@link Map}, it will be recursively cleaned too.<br>
+     * {@link Map}s that are or get empty, will be removed (recursively).<br>
+     */
+    protected void cleanValuesMap(Map<?, ?> map) {
+        for (Object key : map.keySet().toArray()) {
+            Object value = map.get(key);
+
+            if (value instanceof Map) {
+                cleanValuesMap((Map<?, ?>) value);
+            }
+
+            if (value == null || (value instanceof Map && ((Map<?, ?>) value).isEmpty())) {
+                map.remove(key);
+            }
+        }
+    }
 }

Core/src/test/java/com/songoda/core/configuration/yaml/YamlConfigurationTest.java

@@ -139,6 +139,19 @@ class YamlConfigurationTest {
         assertEquals(expectedOutYaml, cfg.toYamlString());
     }
 
+    @Test
+    void testYamlWriterWithNullValue() throws IOException {
+        final YamlConfiguration cfg = new YamlConfiguration();
+        final StringWriter stringWriter = new StringWriter(1);
+
+        cfg.set("null-value", null);
+        cfg.set("nested.null-value", null);
+        cfg.save(stringWriter);
+
+        assertEquals("", stringWriter.toString());
+        assertEquals("", cfg.toYamlString());
+    }
+
     @Test
     void testYamlWriterWithNoData() throws IOException {
         final YamlConfiguration cfg = new YamlConfiguration();
@@ -239,6 +252,16 @@ class YamlConfigurationTest {
         assertEquals("value", cfg.get("primitives.map.key"));
     }
 
+    @Test
+    void testSetterWithNullValue() {
+        final YamlConfiguration cfg = new YamlConfiguration();
+
+        assertNull(cfg.set("foo.bar.null", "not-null-string"));
+        assertEquals("not-null-string", cfg.set("foo.bar.null", null));
+
+        assertNull(cfg.get("foo.bar.null"));
+    }
+
     @Test
     void testGetNonExistingNestedKey() {
         final YamlConfiguration cfg = new YamlConfiguration();