fix and harden BatteryStats update timestamp handling

* updating the SoC or value shall also update the general timestamp, as
  the latter is defined as "any value changed", which includes SoC and
  voltage, of course.
* if the last update is not a valid timestamp at all, the
  updateAvailable method must always return false, obviously.

include/BatteryStats.h

@@ -45,12 +45,12 @@ class BatteryStats {
         void setSoC(float soc, uint8_t precision, uint32_t timestamp) {
             _soc = soc;
             _socPrecision = precision;
-            _lastUpdateSoC = timestamp;
+            _lastUpdateSoC = _lastUpdate = timestamp;
         }
 
         void setVoltage(float voltage, uint32_t timestamp) {
             _voltage = voltage;
-            _lastUpdateVoltage = timestamp;
+            _lastUpdateVoltage = _lastUpdate = timestamp;
         }
 
         String _manufacturer = "unknown";

src/BatteryStats.cpp

@@ -53,6 +53,8 @@ static void addLiveViewAlarm(JsonVariant& root, std::string const& name,
 
 bool BatteryStats::updateAvailable(uint32_t since) const
 {
+    if (_lastUpdate == 0) { return false; } // no data at all processed yet
+
     auto constexpr halfOfAllMillis = std::numeric_limits<uint32_t>::max() / 2;
     return (_lastUpdate - since) < halfOfAllMillis;
 }