Mojo: Fix warnings when generating code for empty struct in java.

mojo/public/tools/bindings/generators/java_templates/struct_definition.tmpl

 {% from "constant_definition.tmpl" import constant_def %}
 {% from "enum_definition.tmpl" import enum_def %}
 
 {% macro encode(variable, kind, offset, bit, level=0) %}
 {% if kind|is_pointer_array_kind %}
 {% set sub_kind = kind.kind %}
 if ({{variable}} == null) {
     encoder{{level}}.encodeNullPointer({{offset}});
 } else {
     org.chromium.mojo.bindings.Encoder encoder{{level + 1}} = encoder{{level}}.encodePointerArray({{variable}}.length, {{offset}});
@@ skipping 60 matching lines @@
 
     public static {{struct|name}} deserialize(org.chromium.mojo.bindings.Message message) {
         return decode(new org.chromium.mojo.bindings.Decoder(message));
     }
 
     public static {{struct|name}} decode(org.chromium.mojo.bindings.Decoder decoder0) {
         if (decoder0 == null) {
             return null;
         }
         {{struct|name}} result = new {{struct|name}}();
+{% if not struct.bytes %}
+        decoder0.readDataHeader();
+{% else %}
         DataHeader mainDataHeader = decoder0.readDataHeader();
+{% endif %}
 {% for byte in struct.bytes %}
 {%   for packed_field in byte.packed_fields %}
         if (mainDataHeader.numFields > {{packed_field.ordinal}}) {
             {{decode('result.' ~ packed_field.field|name, packed_field.field.kind, 8+packed_field.offset, packed_field.bit)|indent(12)}}
         }
 {%   endfor %}
 {% endfor %}
         return result;
     }
 
     @Override
     protected final void encode(org.chromium.mojo.bindings.Encoder encoder) {
+{% if not struct.bytes %}
+        encoder.getEncoderAtDataOffset(DEFAULT_STRUCT_INFO);
+{% else %}
         org.chromium.mojo.bindings.Encoder encoder0 = encoder.getEncoderAtDataOffset(DEFAULT_STRUCT_INFO);
+{% endif %}
 {% for byte in struct.bytes %}
 {%   for packed_field in byte.packed_fields %}
         {{encode(packed_field.field|name, packed_field.field.kind, 8+packed_field.offset, packed_field.bit)|indent(8)}}
 {%   endfor %}
 {% endfor %}
     }
 }
 {% endmacro %}