Also skip SPL check for empty SPL strings

Test: th
Bug: 244525514


Change-Id: Ia5b94d115003caafa36c9ec46d167af930283752
diff --git a/payload_consumer/delta_performer.cc b/payload_consumer/delta_performer.cc
index 382f002..45103e6 100644
--- a/payload_consumer/delta_performer.cc
+++ b/payload_consumer/delta_performer.cc
@@ -395,6 +395,9 @@
   if (!manifest_.has_security_patch_level()) {
     return;
   }
+  if (manifest_.security_patch_level().empty()) {
+    return;
+  }
   const auto new_spl = manifest_.security_patch_level();
   const auto current_spl =
       android::base::GetProperty("ro.build.version.security_patch", "");