[poincare/zoom] Handle undefined functions

Rework the logic so that :
  - an undefined function will be displayed with a clealry defined
    null range.
  - display both an undefined function and a correct function at the
    same time will not affect the correct range.

Change-Id: Ife9dc0d2ace667cab5a6b8826347078fca33e4d5

apps/shared/interactive_curve_view_range.cpp

@@ -134,6 +134,14 @@ void InteractiveCurveViewRange::setDefault() {
   normalize();
 }
 
+void InteractiveCurveViewRange::setNullRange() {
+  m_xRange.setMin(- Range1D::k_default);
+  setXMax(Range1D::k_default);
+  m_yRange.setMin(0);
+  m_yRange.setMax(0);
+  normalize();
+}
+
 void InteractiveCurveViewRange::centerAxisAround(Axis axis, float position) {
   if (std::isnan(position)) {
     return;