HAL_EMPTY: prevent valgrind errors

zero I2C transfers

libraries/AP_HAL_Empty/I2CDriver.cpp

@@ -18,11 +18,21 @@ uint8_t EmptyI2CDriver::writeRegisters(uint8_t addr, uint8_t reg,
 {return 0;}
 
 uint8_t EmptyI2CDriver::read(uint8_t addr, uint8_t len, uint8_t* data)
-{return 0;}
+{
+    memset(data, 0, len);
+    return 0;
+}
 uint8_t EmptyI2CDriver::readRegister(uint8_t addr, uint8_t reg, uint8_t* data)
-{return 0;}
+{
+    *data = 0;
+    return 0;
+}
+
 uint8_t EmptyI2CDriver::readRegisters(uint8_t addr, uint8_t reg,
-                              uint8_t len, uint8_t* data)
-{return 0;}
+                                      uint8_t len, uint8_t* data)
+{
+    memset(data, 0, len);    
+    return 0;
+}
 
 uint8_t EmptyI2CDriver::lockup_count() {return 0;}