[TYPO3] if-conditions / override

[PEF]

Hey,

I'm having some problems with an if-condition. I've tried several 
solutions without any succes.In short I want to create a tag containing 
an image and a text. If not the 'nav_title' is empty - use it. Else use 
the 'title'.


My ts-code looks somethis like this:
lib.tabUpper = IMAGE
lib.tabUpper.file = GIFBUILDER
lib.tabUpper.file {
	XY = 158, 21
	backColor = black
	10 = IMAGE
	10 {
		file = fileadmin/sys/images/tab_upper.gif
		file.width = 158	
	}
	
	20 = TEXT
	20 {
		text = {page:nav_title}
                 if {
			isFalse.field = nav_title
			text = {page:title}
		}
		text.insertData = 1
		fontSize = 12
		fontColor = black
     		fontFile = fileadmin/sys/fonts/HandelGo.ttf
     		offset = 0,17
     		align = center
     		niceText = 0
     		iterations = 1
	}
}



Regards
Peter Falk