Gui: Make StyleParameterManager::resolve return optional result

This changes the resolve method of style parameter manager to return
optional intead of definitive result. In reality tokens may not be
defioned correctly and optionals provide a good way to nicely handle
defaults.

src/Gui/StyleParameters/ParameterManager.cpp

@@ -260,12 +260,16 @@ std::string ParameterManager::replacePlaceholders(const std::string& expression,
         QString::fromStdString(expression),
         [&](const QRegularExpressionMatch& match) {
             auto tokenName = match.captured(1).toStdString();
-
             auto tokenValue = resolve(tokenName, context);
-            context.visited.erase(tokenName);
 
-            return QString::fromStdString(tokenValue.toString());
-        }
+            if (!tokenValue) {
+                Base::Console().warning("Requested non-existent style parameter token '%s'.\n", tokenName);
+                return QStringLiteral("");
+            }
+
+            context.visited.erase(tokenName);
+            return QString::fromStdString(tokenValue->toString());
+    }
     ).toStdString();
     // clang-format on
 }
@@ -293,18 +297,18 @@ std::optional<std::string> ParameterManager::expression(const std::string& name)
     return {};
 }
 
-Value ParameterManager::resolve(const std::string& name, ResolveContext context) const
+std::optional<Value> ParameterManager::resolve(const std::string& name,
+                                               ResolveContext context) const
 {
     std::optional<Parameter> maybeParameter = this->parameter(name);
 
     if (!maybeParameter) {
-        Base::Console().warning("Requested non-existent design token '%s'.\n", name);
-        return std::string {};
+        return std::nullopt;
     }
 
     if (context.visited.contains(name)) {
-        Base::Console().warning("The design token '%s' contains circular-reference.\n", name);
-        return expression(name).value_or(std::string {});
+        Base::Console().warning("The style parameter '%s' contains circular-reference.\n", name);
+        return expression(name);
     }
 
     const Parameter& token = *maybeParameter;