fix: set default version for xml2 checker to UNKNOWN (Fixes #1517)

cve_bin_tool/checkers/xml2.py

@@ -28,7 +28,7 @@ class Xml2Checker(Checker):
     @staticmethod
     def guess_xml2_version(lines):
         """Guesses the xml2 version from the file contents"""
-        new_guess = ""
+        new_guess = "UNKNOWN"
         pattern1 = re.compile(r"/libxml2-([0-9]+\.[0-9]+\.[0-9]+)/")
         pattern2 = re.compile(r"\\libxml2-([0-9]+\.[0-9]+\.[0-9]+)\\")
         # fedora 29 string looks like libxml2.so.2.9.8-2.9.8-4.fc29.x86_64.debug