Add unit tests for simulator

Make sure the simulator succeeds executing common non-A/B update
functions.

Bug: 131911365
Test: run unit tests
Change-Id: I520ce6a8827539b88a9e36f9e67eec30d8b586d4
diff --git a/updater/target_files.cpp b/updater/target_files.cpp
index 93540b2..092edb9 100644
--- a/updater/target_files.cpp
+++ b/updater/target_files.cpp
@@ -132,6 +132,11 @@
     return false;
   }
 
+  if (entry.uncompressed_length == 0) {
+    content->clear();
+    return true;
+  }
+
   content->resize(entry.uncompressed_length);
   if (auto extract_err = ExtractToMemory(
           handle_, &entry, reinterpret_cast<uint8_t*>(&content->at(0)), entry.uncompressed_length);