ExtensionUtilTests added tests for no extension and dot files.

api/src/test/groovy/com/jessebrault/ssg/util/ExtensionUtilTests.groovy

@@ -3,6 +3,7 @@ package com.jessebrault.ssg.util
 import org.junit.jupiter.api.Test
 
 import static org.junit.jupiter.api.Assertions.assertEquals
+import static org.junit.jupiter.api.Assertions.assertNull
 
 final class ExtensionUtilTests {
 
@@ -42,6 +43,16 @@ final class ExtensionUtilTests {
             assertEquals('.txt', ExtensionUtil.getExtension('test.test.txt'))
         }
 
+        @Test
+        void noExtensionReturnsNull() {
+            assertNull(ExtensionUtil.getExtension('NO_EXTENSION'))
+        }
+
+        @Test
+        void dotFileReturnsNull() {
+            assertNull(ExtensionUtil.getExtension('.dot_file'))
+        }
+
     }
 
 }