PwBaseWorkChain: make magnetism from overrides absolute

When using the `get_builder_from_protocol()` method from the
`PwBaseWorkChain`, setting `spin_type` to `COLLINEAR` while not
specifying the `initial_magnetic_moments` will result in a set of
default magnetic moments being used, based on the `magnetization.yaml`
file. This means that even when `SYSTEM.starting_magnetization` is
specified in the `overrides`, these are simply ignored in favor of the
defaults.

Here we make the `overrides` _absolute_, i.e. the
`starting_magnetization` is only set to the default in case it is not
specified in the `overrides`. This is once again based on the principle
that we should avoid quietly overriding inputs that the user explicitly
specified.

aiida_quantumespresso/workflows/pw/base.py

@@ -205,10 +205,11 @@ def get_builder_from_protocol(
             parameters['SYSTEM'].pop('smearing')
 
         if spin_type is SpinType.COLLINEAR:
-            starting_magnetization = get_starting_magnetization(structure, pseudo_family, initial_magnetic_moments)
-
             parameters['SYSTEM']['nspin'] = 2
-            parameters['SYSTEM']['starting_magnetization'] = starting_magnetization
+            if 'starting_magnetization' not in parameters['SYSTEM']:
+                starting_magnetization = get_starting_magnetization(structure, pseudo_family, initial_magnetic_moments)
+
+                parameters['SYSTEM']['starting_magnetization'] = starting_magnetization
 
         # pylint: disable=no-member
         builder = cls.get_builder()