Fix exception on loading empty file in YamlConfiguration

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

@@ -183,9 +183,12 @@ public class YamlConfiguration implements IConfiguration, HeaderCommentable, Nod
     @Override
     public void load(Reader reader) {
         Object yamlData = this.yaml.load(reader);
+        if (yamlData == null) {
+            yamlData = Collections.emptyMap();
+        }
 
         if (!(yamlData instanceof Map)) {
-            throw new IllegalStateException("The YAML file does not have the expected tree structure");
+            throw new IllegalStateException("The YAML file does not have the expected tree structure: " + yamlData.getClass().getName());
         }
 
         synchronized (this.values) {

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

@@ -103,6 +103,16 @@ class YamlConfigurationTest {
         assertEquals(3, ((List<?>) cfg.get("primitives.set")).size());
     }
 
+    @Test
+    void testYamlParserWithEmptyFile() {
+        final YamlConfiguration cfg = new YamlConfiguration();
+        cfg.load(new StringReader(""));
+        assertTrue(cfg.getKeys("").isEmpty());
+
+        cfg.load(new StringReader("\n"));
+        assertTrue(cfg.getKeys("").isEmpty());
+    }
+
     @Test
     void testYamlParserWithDuplicateKeys() {
         assertThrowsExactly(DuplicateKeyException.class,
@@ -524,7 +534,7 @@ class YamlConfigurationTest {
         IllegalStateException exception = assertThrowsExactly(IllegalStateException.class,
                 () -> cfg.load(new StringReader("Hello world")));
 
-        assertEquals("The YAML file does not have the expected tree structure", exception.getMessage());
+        assertEquals("The YAML file does not have the expected tree structure: java.lang.String", exception.getMessage());
     }
 
     @Test