Adding just one character will make your property return nothing instead an error:
concatenation ", " of names of keys of keys “HKEY_LOCAL_MACHINE\SOFTWARE\JavaSoft\Java Runtime Environment” of registry
If you want it to return “Not Installed” instead nothing, you can wrap it in an if statement:
if (exists keys of keys “HKEY_LOCAL_MACHINE\SOFTWARE\JavaSoft\Java Runtime Environment” of registry)
then (concatenation ", " of names of keys of keys “HKEY_LOCAL_MACHINE\SOFTWARE\JavaSoft\Java Runtime Environment” of registry)
else (“Not Installed”)