Fixes #36741 - Check Ubuntu version more explicitly

If no minor version is defined, the function minor.to_i makes it a "0".
Accordingly, Ubuntu 20.04 is interpreted as "no subiquity/Autoinstall" - but,
if no minor version is given, Foreman should implicitly assume "latest"
(which supports Autoinstall in fact).

app/models/operatingsystems/debian.rb

@@ -81,7 +81,7 @@ def guess_os
   def is_subiquity?
     return false if guess_os != "ubuntu"
     return false if major.to_i < 20
-    return false if major.to_i == 20 && minor.to_i <= 2
+    return false if major.to_i == 20 && minor.present? && minor.to_i <= 2
     true # Ubuntu release 20.04.3 or newer
   end