[Cleanup] Use constant reference and check for empty string properly in dbcore.cpp (#3203)

# Notes
- Passing by constant reference is more performant.
- Checking for empty string with `!= '\0'` is more performant.
- https://pvs-studio.com/en/docs/warnings/v805/
- https://pvs-studio.com/en/docs/warnings/v813/

common/dbcore.h

@@ -35,7 +35,7 @@ public:
 	const std::string &GetOriginHost() const;
 	void SetOriginHost(const std::string &origin_host);
 
-	bool DoesTableExist(std::string table_name);
+	bool DoesTableExist(const std::string& table_name);
 
 	void SetMySQL(const DBcore &o)
 	{