Skip writing empty SPL string into update manifest

If field is empty, no need to waste space storing it in protobuf.

Test: generate OTA, make sure SPL field is unset
Bug: 244525514
Change-Id: I43630efaf7c082541af786657110c345c4f6ee17
diff --git a/payload_generator/payload_file.cc b/payload_generator/payload_file.cc
index 0ef747a..f404c79 100644
--- a/payload_generator/payload_file.cc
+++ b/payload_generator/payload_file.cc
@@ -66,7 +66,9 @@
   manifest_.set_minor_version(config.version.minor);
   manifest_.set_block_size(config.block_size);
   manifest_.set_max_timestamp(config.max_timestamp);
-  manifest_.set_security_patch_level(config.security_patch_level);
+  if (!config.security_patch_level.empty()) {
+    manifest_.set_security_patch_level(config.security_patch_level);
+  }
 
   if (config.target.dynamic_partition_metadata != nullptr)
     *(manifest_.mutable_dynamic_partition_metadata()) =
diff --git a/payload_generator/payload_properties_unittest.cc b/payload_generator/payload_properties_unittest.cc
index 0d62681..0ff364f 100644
--- a/payload_generator/payload_properties_unittest.cc
+++ b/payload_generator/payload_properties_unittest.cc
@@ -104,9 +104,9 @@
       "{"
       R"("is_delta":true,)"
       R"("metadata_signature":"",)"
-      R"("metadata_size":168,)"
-      R"("sha256_hex":"6rXHDjFO8k8mNtIbLhimWOifecVI1Ts230Ia1DyNuPY=",)"
-      R"("size":214,)"
+      R"("metadata_size":165,)"
+      R"("sha256_hex":"cV7kfZBH3K0B6QJHxxykDh6b6x0WgVOmc63whPLOy7U=",)"
+      R"("size":211,)"
       R"("version":2)"
       "}";
   string json;
@@ -118,10 +118,10 @@
 // Validate the hash of file and metadata are within the output.
 TEST_F(PayloadPropertiesTest, GetPropertiesAsKeyValueTestHash) {
   constexpr char kKeyValueProperties[] =
-      "FILE_HASH=6rXHDjFO8k8mNtIbLhimWOifecVI1Ts230Ia1DyNuPY=\n"
-      "FILE_SIZE=214\n"
-      "METADATA_HASH=wq2nRZ7o/aqEeVWcc2Z+bebLYEI8quPEnXHlyLtdW9Y=\n"
-      "METADATA_SIZE=168\n";
+      "FILE_HASH=cV7kfZBH3K0B6QJHxxykDh6b6x0WgVOmc63whPLOy7U=\n"
+      "FILE_SIZE=211\n"
+      "METADATA_HASH=aEKYyzJt2E8Gz8fzB+gmekN5mriotZCSq6R+kDfdeV4=\n"
+      "METADATA_SIZE=165\n";
   string key_value;
   EXPECT_TRUE(PayloadProperties{payload_file_.path()}.GetPropertiesAsKeyValue(
       &key_value));