Fix grid current formula generation

Add the formula operator if and only if there is a component to add.
Otherwise, this was adding a `+` operator, but not adding a component
after the `+`.

Signed-off-by: Daniel Zullo <[email protected]>

Author: daniel-zullo-frequenz

src/frequenz/sdk/timeseries/formula_engine/_formula_generators/_grid_current_formula.py

@@ -52,9 +52,6 @@ def _gen_phase_formula(
         # generate a formula that just adds values from all components that are
         # directly connected to the grid.
         for idx, comp in enumerate(grid_successors):
-            if idx > 0:
-                builder.push_oper("+")
-
             # When inverters or ev chargers produce `None` samples, those
             # inverters are excluded from the calculation by treating their
             # `None` values as `0`s.
@@ -71,6 +68,9 @@ def _gen_phase_formula(
             else:
                 continue
 
+            if idx > 0:
+                builder.push_oper("+")
+
             builder.push_component_metric(
                 comp.component_id, nones_are_zeros=nones_are_zeros
             )