(Merge 3.2) Issue #13545: Fix platform.libc_version() is the SO version is missing

Lib/platform.py

@@ -186,7 +186,7 @@ def libc_ver(executable=sys.executable,lib='',version='',
         elif so:
             if lib != 'glibc':
                 lib = 'libc'
-                if soversion > version:
+                if soversion and soversion > version:
                     version = soversion
                 if threads and version[-len(threads):] != threads:
                     version = version + threads