Config Attribute Value: added write_if_not_empty helper

Takes a string and assigns a value only if that string is not empty.

src/config_attribute_value.cpp

@@ -235,6 +235,13 @@ config_attribute_value& config_attribute_value::operator=(const t_string& v)
 	return *this;
 }
 
+void config_attribute_value::write_if_not_empty(const std::string& v)
+{
+	if(!v.empty()) {
+		*this = v;
+	}
+}
+
 bool config_attribute_value::to_bool(bool def) const
 {
 	if(const yes_no* p = boost::get<const yes_no>(&value_))

src/config_attribute_value.hpp

@@ -177,6 +177,10 @@ class config_attribute_value
 	{
 		return operator=(T::enum_to_string(v));
 	}
+
+	/** Calls @ref operator=(const std::string&) if @a v is not empty. */
+	void write_if_not_empty(const std::string& v);
+
 	// Extracting as a specific type:
 	bool to_bool(bool def = false) const;
 	int to_int(int def = 0) const;