Fix formatting bug for zero values (#503)

Also add tests for zero and negative values.

The base branch for this PR is v0.22.x, so that we can release a patch
release v0.22.1 immediately. Once this is merged, we can merge v0.22.x
into v0.x.x.

Author: shsms

src/frequenz/sdk/timeseries/_quantities.py

@@ -164,7 +164,7 @@ def __format__(self, __format_spec: str) -> str:
             return f"{self._base_value:.{precision}f}"
 
         abs_value = abs(self._base_value)
-        exponent = math.floor(math.log10(abs_value))
+        exponent = math.floor(math.log10(abs_value)) if abs_value else 0
         unit_place = exponent - exponent % 3
         if unit_place < min(self._exponent_unit_map):
             unit = self._exponent_unit_map[min(self._exponent_unit_map.keys())]

tests/timeseries/test_quantities.py

@@ -84,6 +84,11 @@ def test_string_representation() -> None:
     assert f"{Fz1(1.024445, exponent=-12)}" == "0 Hz"
     assert f"{Fz2(1.024445, exponent=-12)}" == "0 Hz"
 
+    assert f"{Fz1(0)}" == "0 Hz"
+
+    assert f"{Fz1(-20)}" == "-20 Hz"
+    assert f"{Fz1(-20000)}" == "-20 kHz"
+
 
 def test_addition_subtraction() -> None:
     """Test the addition and subtraction of the quantities."""