Make `Fields` interpret null values as empty strings (#12024)

#12018 make Fields interpret null values as empty strings

Signed-off-by: Ludovic Orban <lorban@bitronix.be>

jetty-core/jetty-util/src/main/java/org/eclipse/jetty/util/Fields.java

@@ -191,7 +191,7 @@ public class Fields implements Iterable<Fields.Field>
     public void put(String name, String value)
     {
         // Preserve the case for the field name
-        Field field = new Field(name, value);
+        Field field = new Field(name, StringUtil.nonNull(value));
         fields.put(name, field);
     }
 
@@ -222,9 +222,9 @@ public class Fields implements Iterable<Fields.Field>
         {
             if (f == null)
                 // Preserve the case for the field name
-                return new Field(name, value);
+                return new Field(name, StringUtil.nonNull(value));
             else
-                return new Field(f.getName(), f.getValues(), value);
+                return new Field(f.getName(), f.getValues(), StringUtil.nonNull(value));
         });
     }
 
@@ -246,9 +246,9 @@ public class Fields implements Iterable<Fields.Field>
             fields.compute(name, (k, f) ->
             {
                 if (f == null)
-                    return new Field(name, List.of(values));
+                    return new Field(name, StringUtil.toListNonNull(values));
                 else
-                    return new Field(f.getName(), f.getValues(), List.of(values));
+                    return new Field(f.getName(), f.getValues(), StringUtil.toListNonNull(values));
             });
         }
     }

jetty-core/jetty-util/src/main/java/org/eclipse/jetty/util/StringUtil.java

@@ -540,6 +540,23 @@ public class StringUtil
         return s;
     }
 
+    /**
+     * Convert an array of strings to a list of non-null strings.
+     *
+     * @param strings the array
+     * @return The list of non-null strings.
+     * @see #nonNull(String)
+     */
+    public static List<String> toListNonNull(String... strings)
+    {
+        List<String> result = new ArrayList<>(strings.length);
+        for (String s : strings)
+        {
+            result.add(nonNull(s));
+        }
+        return result;
+    }
+
     public static boolean equals(String s, char[] buf, int offset, int length)
     {
         if (s.length() != length)

jetty-core/jetty-util/src/test/java/org/eclipse/jetty/util/FieldsTest.java

@@ -82,4 +82,18 @@ public class FieldsTest
 
         assertThat(set, containsInAnyOrder("x", "y", "z"));
     }
+
+    @Test
+    public void testNullValues()
+    {
+        Fields fields = new Fields();
+        fields.add("x", (String)null);
+        fields.add("y", "1", null, "2");
+        fields.put("z", null);
+
+        assertThat(fields.getSize(), equalTo(3));
+        assertThat(fields.getValues("x"), contains(""));
+        assertThat(fields.getValues("y"), contains("1", "", "2"));
+        assertThat(fields.getValues("z"), contains(""));
+    }
 }