Fix warning for rkp_factory_extraction_tool

The empty string was handled as a value. Also, the unit test that tested
this functionality was fixed.

Bug: 377389989
Test: atest system/security/provisioner
Test: m rkp_factory_extraction_tool
Test: adb push rkp_factory_extraction_tool /data
Test: adb shell adb shell /data/rkp_factory_extraction_tool
Test: The above command does not produce a warning.
Change-Id: Ifd12bb43956be00efce0b45656da6d65e19c02fa
diff --git a/provisioner/rkp_factory_extraction_lib.h b/provisioner/rkp_factory_extraction_lib.h
index 2c1e2ff..3515f48 100644
--- a/provisioner/rkp_factory_extraction_lib.h
+++ b/provisioner/rkp_factory_extraction_lib.h
@@ -27,6 +27,7 @@
 #include <vector>
 
 // Parse a comma-delimited string.
+// Ignores any empty strings.
 std::unordered_set<std::string> parseCommaDelimited(const std::string& input);
 
 // Challenge size must be between 32 and 64 bytes inclusive.