this is not supposed to return null (#3998)

see https://javaee.github.io/javaee-spec/javadocs/javax/servlet/http/Part.html#getHeaders-java.lang.String-
this fix related tck test

Signed-off-by: olivier lamy <oliver.lamy@gmail.com>

jetty-http/src/main/java/org/eclipse/jetty/http/MultiPartFormInputStream.java

@@ -205,7 +205,8 @@ public class MultiPartFormInputStream
         @Override
         public Collection<String> getHeaders(String name)
         {
-            return _headers.getValues(name);
+            Collection<String> headers = _headers.getValues(name);
+            return headers == null ? Collections.emptyList():headers;
         }
 
         @Override

jetty-http/src/test/java/org/eclipse/jetty/http/MultiPartFormInputStreamTest.java

@@ -786,6 +786,8 @@ public class MultiPartFormInputStreamTest
         assertThat(stuff.getContentType(), is("text/plain"));
         assertThat(stuff.getHeader("Content-Type"), is("text/plain"));
         assertThat(stuff.getHeaders("content-type").size(), is(1));
+        assertNotNull(stuff.getHeaders("non existing part"));
+        assertThat(stuff.getHeaders("non existing part").size(), is(0));
         assertThat(stuff.getHeader("content-disposition"), is("form-data; name=\"stuff\"; filename=\"" + filename + "\""));
         assertThat(stuff.getHeaderNames().size(), is(2));
         assertThat(stuff.getSize(), is(51L));