fix [-Wunused-parameter] warnings

they appear when using `Wextra` flag

```
lib/DnAppLogger/src/DnApp/Logger/Endpoint/NullLogger.h:14:34: warning: unused parameter 'level' [-Wunused-parameter]
        auto log(const LOG_LEVEL level, const char* const message) -> void override {
                                 ^
lib/DnAppLogger/src/DnApp/Logger/Endpoint/NullLogger.h:14:59: warning: unused parameter 'message' [-Wunused-parameter]
        auto log(const LOG_LEVEL level, const char* const message) -> void override {
                                                          ^
lib/DnAppArduino/src/DnApp/Arduino/Logger/Endpoint/WithArduinoStringNullLogger.h:15:34: warning: unused parameter 'level' [-Wunused-parameter]
        auto log(const LOG_LEVEL level, const char* const message) -> void override {
                                 ^
lib/DnAppArduino/src/DnApp/Arduino/Logger/Endpoint/WithArduinoStringNullLogger.h:15:59: warning: unused parameter 'message' [-Wunused-parameter]
        auto log(const LOG_LEVEL level, const char* const message) -> void override {
                                                          ^
```

README.md

@@ -262,5 +262,13 @@ Code style, conventions, decisions
 * `// todo:[2137] Description of what is to be done`
   – the number is for easier grep,
     when the same kind of issue appears in multiple places.
+* `static_cast` to `void` when function arguments are not being used.
+  I know of 3 possible solutions:
+    * A.) `static_cast<void>(unusedParameter);`
+          – the preferred C++ way, that we're doing,
+    * B.) `(void) unusedParameter;`
+          – the "old" C–style casting, but `A.)` is more precise,
+    * C.) remove `unusedParameter` var from the function arguments
+          – but keeping the name better explains what it represents.
 
 ---

lib/DnAppArduino/src/DnApp/Arduino/Logger/Endpoint/WithArduinoStringNullLogger.h

@@ -13,6 +13,10 @@ namespace DnApp::Arduino::Logger::Endpoint {
         using WithArduinoStringLogger::log;
 
         auto log(const LOG_LEVEL level, const char* const message) -> void override {
+            // fix `[-Wunused-parameter]` warnings:
+            static_cast<void>(level);
+            static_cast<void>(message);
+
             // do nothing
         };
     };

lib/DnAppLogger/src/DnApp/Logger/Endpoint/NullLogger.h

@@ -12,6 +12,10 @@ namespace DnApp::Logger::Endpoint {
         using Logger::log;
 
         auto log(const LOG_LEVEL level, const char* const message) -> void override {
+            // fix `[-Wunused-parameter]` warnings:
+            static_cast<void>(level);
+            static_cast<void>(message);
+
             // do nothing
         };
     };