IF statement in Rscript / null variables in Rscript

Hi,

I want to perform a regression analysis with R script. I used mtcars dataset as test data. My regression formula is basically: lmod<-lm(mpg~cyl+wt+hp)

Also wanted to enable users to choose which variables to use as y (predicted) variable. To do so created a parameter as YVariable which works on a list query consists of column (variable) names. So predicted variable is set via parameter.

Note: I put a null row in list query. 

If YVVariable(mpg) formula is;

lmod<-lm("&Text.From(YVariable)&"~cyl+wt+hp,mtcars)

lmod<-lm(mpg~cyl+wt+hp,mtcars)

And it works!

My next challange is to do the same thing for independent variables (such as cyl, wt, hp ...)

To do so I created parameters called Independent1,2,3...

lmod<-lm("&Text.From(YVariable)&"~"&Text.From(Independent1)&"+"&Text.From(Independent2)&"+"&Text.From(Independent3)&",mtcars)

In this case if 

Independent1(hp)

Independent1(wt)

Independent1(cyl) formula will be lmod<-lm(mpg~cyl+wt+hp)

But I want to be able to set the formula as lmod<-lm(mpg~cyl+wt) (without hp) In this case Independent3() should be null.

But if one of the Independent parameters is null - Independent1() - BI returns following error,

Expression.Error: We cannot convert the value null to type Text.
Details:
Value=
Type=Type

What could be Done? Any ideas?

Thanks!