Spreadsheet: do not create string property for empty cell

Otherwise may cause massive slow down when, for example, the user
changes the entire row/column color, which results in large amount of
empty cells.

src/Mod/Spreadsheet/App/Cell.cpp

@@ -288,6 +288,8 @@ void Cell::setContent(const char * value)
     clearException();
     if (value) {
         if (owner->sheet()->isRestoring()) {
+            if (value[0] == '\0' || (value[0] == '\'' && value[1] == '\0'))
+                return;
             expression.reset(new App::StringExpression(owner->sheet(), value));
             setUsed(EXPRESSION_SET, true);
             return;
@@ -302,7 +304,10 @@ void Cell::setContent(const char * value)
             }
         }
         else if (*value == '\'') {
-            newExpr = std::make_unique<App::StringExpression>(owner->sheet(), value + 1);
+            if (value[1] == '\0')
+                value = nullptr;
+            else
+                newExpr = std::make_unique<App::StringExpression>(owner->sheet(), value + 1);
         }
         else if (*value != '\0') {
             // check if value is just a number

src/Mod/Spreadsheet/App/Sheet.cpp

@@ -683,10 +683,12 @@ void Sheet::updateProperty(CellAddress key)
         else {
             std::string s;
 
-            if (cell->getStringContent(s))
+            if (cell->getStringContent(s) && !s.empty())
                 output.reset(new StringExpression(this, s));
-            else
-                output.reset(new StringExpression(this, ""));
+            else {
+                this->removeDynamicProperty(key.toString().c_str());
+                return;
+            }
         }
 
         /* Eval returns either NumberExpression or StringExpression, or