Combining Data Across Multiple Data Fields

I am trying to create a common data field based on multiple columns below. When I try to use nvl i am getting blank values in the common field and its not getting picked up..The input and output tables are below:

Input table

model-manufacture	model -CAM	MODEL -schedule
	CX190
	NM890	NM890
JK1231
LO121	LO121
		VB1231
OPE12		ope12
jk7812

Output table

model-common	model-manufacture	model -CAM	MODEL -schedule
CX190		CX190
NM890		NM890	NM890
JK1231	JK1231
LO121	LO121	LO121
VB1231			VB1231
ope12	OPE12		ope12
jk7812	jk7812

Query Used

select nvl(model-manufacture,model-CAM,MODEL-schedule) as model-common
from input

Answer

On wa is to replace empty strings with NULL with nullif

But it would be best, to save NULL

select nvl(nullif(TRIM(model-manufacture),'')
,nullif(TRIM(model-CAM),''),nullif(TRIM(MODEL-schedule),'')) as model-common
from input