Add null GPIO pin (#6611)

esphome/core/gpio.h

@@ -62,6 +62,24 @@ class GPIOPin {
   virtual bool is_internal() { return false; }
 };
 
+/**
+ * A pin to replace those that don't exist.
+ */
+class NullPin : public GPIOPin {
+ public:
+  void setup() override {}
+
+  void pin_mode(gpio::Flags _) override {}
+
+  bool digital_read() override { return false; }
+
+  void digital_write(bool _) override {}
+
+  std::string dump_summary() const override { return {"Not used"}; }
+};
+
+static GPIOPin *const NULL_PIN = new NullPin();
+
 /// Copy of GPIOPin that is safe to use from ISRs (with no virtual functions)
 class ISRInternalGPIOPin {
  public: