Resolved NPE and updated unit tests

Author: stevespringett

src/main/java/com/github/packageurl/PackageURL.java

@@ -211,7 +211,7 @@ public String getVersion() {
      * @since 1.0.0
      */
     public Map<String, String> getQualifiers() {
-        return Collections.unmodifiableMap(qualifiers);
+        return (qualifiers != null)? Collections.unmodifiableMap(qualifiers) : null;
     }
 
     /**

src/test/java/com/github/packageurl/PackageURLTest.java

@@ -92,7 +92,9 @@ public void testConstructorParsing() throws Exception {
             Assert.assertEquals(name, purl.getName());
             Assert.assertEquals(version, purl.getVersion());
             Assert.assertEquals(subpath, purl.getSubpath());
-            if (qualifiers != null) {
+            if (qualifiers == null) {
+                Assert.assertNull(purl.getQualifiers());
+            } else {
                 Assert.assertNotNull(purl.getQualifiers());
                 Assert.assertEquals(qualifiers.length(), purl.getQualifiers().size());
                 qualifiers.keySet().forEach((key) -> {