Automator Service that works with and without selection
I don't have an exact answer to your problem but you could perhaps consider a normal applescript that sends the COPY command and checks the contents of the clipboard before and after. Like this... 

# For testing only; Delay so we can switch to the app with text
delay 2

# Clear the clipboard so we have a clean state
set the clipboard to ""


    tell application "System Events"

    # Send the 'Copy' command
    keystroke "c" using command down
    delay 0.2

    # Grub the clipboard contents
    set the_sel to the clipboard

    # Is it empty? We either failed or no selection present
    if the_sel is equal to "" then
        display dialog "No Text selected"
    else
        display dialog "Text was: " &amp; the_sel
    end if
end tell