Return partition size in hex format.

This is required to pass fuzzy_fastboot conformance tests.
Also, allow for zero sized partitions in fuzzy_fastboot.

Test: ./fuzzy_fastboot --gtest_filter=Conformance.Slots
Bug: 117220134

Change-Id: Ifb12994a7796b081215084cb68b37674210aaa12
diff --git a/fastboot/device/variables.cpp b/fastboot/device/variables.cpp
index 2de79b1..a7f8ba8 100644
--- a/fastboot/device/variables.cpp
+++ b/fastboot/device/variables.cpp
@@ -289,7 +289,7 @@
     bool is_zero_length;
     if (LogicalPartitionExists(args[0], device->GetCurrentSlot(), &is_zero_length) &&
         is_zero_length) {
-        *message = "0";
+        *message = "0x0";
         return true;
     }
     // Otherwise, open the partition as normal.