updating utility_meter example to use new template format (#19826)

Co-authored-by: Franck Nijhof <frenck@frenck.nl>

source/_integrations/utility_meter.markdown

@@ -226,22 +226,34 @@ utility_meter:
     cycle: monthly
 ```
 
-Additionally, you can add template sensors to compute daily and monthly total usage.
+Additionally, you can add template sensors to compute daily and monthly total usage. Important note, in these examples,
+we use the `is_number()` [function](/docs/configuration/templating/#numeric-functions-and-filters) to verify the values
+returned from the sensors are numeric. If this evalutes to false, `None` is returned.
 
 {% raw %}
 
 ```yaml
-sensor:
+template:
-  - platform: template
+  - sensor:
-    sensors:
+    - name: 'Daily Energy Total'
-      daily_energy:
+      device_class: energy
-        friendly_name: Daily Energy
+      unit_of_measurement: kWh
-        unit_of_measurement: kWh
+      state: >
-        value_template: "{{ states('sensor.daily_energy_offpeak')|float + states('sensor.daily_energy_peak')|float }}"
+        {% if is_number(states('sensor.daily_energy_offpeak')) and is_number(states('sensor.daily_energy_peak')) %}
-      monthly_energy:
+          {{ (states('sensor.daily_energy_offpeak') + states('sensor.daily_energy_peak')) | float }}
-        friendly_name: Monthly Energy
+        {% else %}
-        unit_of_measurement: kWh
+          None
-        value_template: "{{ states('sensor.monthly_energy_offpeak')|float + states('sensor.monthly_energy_peak')|float }}"
+        {% endif %}
+
+    - name: 'Monthly Energy Total'
+      device_class: energy
+      unit_of_measurement: kWh
+      state: >
+        {% if is_number(states('sensor.monthly_energy_offpeak')) and is_number(states('sensor.monthly_energy_peak')) %}
+          {{ (states('sensor.monthly_energy_offpeak') + states('sensor.monthly_energy_peak')) | float }}
+        {% else %}
+          None
+        {% endif %}
 ```
 
 {% endraw %}