Add solutions category and resolve bug in combine_components

[ka-sarthak]

• [x] Add entry data category for solutions schemas
• When a solute component is added without any volume, the normalization runs into error:

  Traceback (most recent call last):
  File "/opt/venv/lib/python3.11/site-packages/nomad/normalizing/metainfo.py", line 38, in normalize_section
  normalize(archive, logger)
  File "/opt/venv/lib/python3.11/site-packages/nomad_material_processing/solution/general.py", line 514, in normalize
  self.solvents = self.combine_components(self.solvents, logger)
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/venv/lib/python3.11/site-packages/nomad_material_processing/solution/general.py", line 425, in combine_components
  combined_components[comparison_key] = component.m_copy(deep=True)
                                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/venv/lib/python3.11/site-packages/nomad/metainfo/metainfo.py", line 2394, in m_copy
  sub_sections_copy.append(sub_section.m_copy(deep=True, parent=copy))
                           ^^^^^^^^^^^^^^^^^^
  AttributeError: 'NoneType' object has no attribute 'm_copy'

  This is an error from MSection.m_copy method. To be resolved in nomad-lab: https://gitlab.mpcdf.mpg.de/nomad-lab/nomad-FAIR/-/merge_requests/2127