gtPlaceSubVolume: small fix in input handling

Signed-off-by: Nicola Vigano <nicola.vigano@esrf.fr>

zUtil_Imaging/gtPlaceSubVolume.m

@@ -22,13 +22,13 @@ function output = gtPlaceSubVolume(output, input, shift, index, assign_op, use_c
 %     Note 2 - If index is 0, the volume will be copied as it is, otherwise
 %     all the voxels will have value of the index
 
-    if (~exist('use_c_functions', 'var'))
+    if (~exist('use_c_functions', 'var') || isempty(use_c_functions))
         use_c_functions = true;
     end
-    if (~exist('index', 'var'))
+    if (~exist('index', 'var') || isempty(index))
         index = 0;
     end
-    if (~exist('assign_op', 'var'))
+    if (~exist('assign_op', 'var') || isempty(assign_op))
         assign_op = 'assign';
     end