Return zero values for missing wrapper map values in the lite runtime

PiperOrigin-RevId: 995526692

Author: l46kok

common/src/main/java/dev/cel/common/values/ProtoLiteCelValueConverter.java

@@ -311,7 +311,11 @@ private Map.Entry<Object, Object> readSingleMapEntry(
       key = getDefaultCelValue(keyDescriptor);
     }
     if (value == null) {
-      value = getDefaultCelValue(valueDescriptor);
+      // Map values have no presence, so a missing message (even a wrapper) is empty, not null.
+      value =
+          valueDescriptor.getJavaType().equals(JavaType.MESSAGE)
+              ? readMessageField(ByteString.EMPTY, valueDescriptor.getFieldProtoTypeName())
+              : getDefaultCelValue(valueDescriptor);
     }
 
     return new AbstractMap.SimpleImmutableEntry<>(key, value);

common/src/test/java/dev/cel/common/values/ProtoLiteCelValueConverterTest.java

@@ -587,6 +587,8 @@ private enum ReadSingleFieldTestCase {
     REPEATED_INT32(TestAllTypes.REPEATED_INT32_FIELD_NUMBER, ImmutableList.of(1L, 2L)),
     MAP_STRING_STRING(
         TestAllTypes.MAP_STRING_STRING_FIELD_NUMBER, ImmutableMap.of("k", "v", "k2", "v2", "", "")),
+    MAP_STRING_INT64_WRAPPER(
+        TestAllTypes.MAP_STRING_INT64_WRAPPER_FIELD_NUMBER, ImmutableMap.of("", 0L)),
     SPLIT_DURATION(
         TestAllTypes.SINGLE_DURATION_FIELD_NUMBER, java.time.Duration.ofSeconds(10, 500));
 
@@ -630,6 +632,8 @@ public void readSingleField_skipsOtherFieldsAndDecodesTarget(
     extraWireCos.writeByteArray(TestAllTypes.REPEATED_INT32_FIELD_NUMBER, new byte[0]);
     extraWireCos.writeByteArray(
         TestAllTypes.MAP_STRING_STRING_FIELD_NUMBER, mapEntryWithUnknownOut.toByteArray());
+    extraWireCos.writeByteArray(
+        TestAllTypes.MAP_STRING_INT64_WRAPPER_FIELD_NUMBER, mapEntryWithUnknownOut.toByteArray());
     extraWireCos.flush();
     ByteString bytes =
         part1