[poincare/expression] Use extractUnits instead of hasUnit when possible

poincare/src/expression.cpp

@@ -350,7 +350,7 @@ Expression Expression::defaultHandleUnitsInChildren() {
   // Generically, an Expression does not accept any Unit in its children.
   const int childrenCount = numberOfChildren();
   for (int i = 0; i < childrenCount; i++) {
-    if (childAtIndex(i).hasUnit()) {
+    if (!childAtIndex(i).extractUnits().isUninitialized()) {
       return replaceWithUndefinedInPlace();
     }
   }

poincare/src/power.cpp

@@ -392,7 +392,7 @@ Expression Power::shallowReduce(ExpressionNode::ReductionContext reductionContex
 
   // Step 1: Handle the units
   {
-    if (index.hasUnit()) {
+    if (!index.extractUnits().isUninitialized()) {
       // There must be no unit in the exponent
       return replaceWithUndefinedInPlace();
     }

poincare/src/unit_convert.cpp

@@ -59,7 +59,7 @@ Expression UnitConvert::shallowReduce(ExpressionNode::ReductionContext reduction
   // Divide the left member by the new unit
   Expression division = Division::Builder(childAtIndex(0), finalUnit.clone());
   division = division.reduce(reductionContext);
-  if (division.hasUnit()) {
+  if (!division.extractUnits().isUninitialized()) {
     // The left and right members are not homogeneous
     return replaceWithUndefinedInPlace();
   }