Fix type declaration in group_concat and blank_nulls

[acwhite211]

Fixes #7423

Resolve sqlalchemy compiling issues related to the group_concat and blank_nulls by making their definitions more friendly to sqlachlemy 1.4. The main way to do this is my setting type = String(), which gives the expression a real TypeEngine, so the compiler/comparator path won’t hit None/NullType.

Checklist

• Self-review the PR after opening it to make sure the changes look good and
  self-explanatory (or properly documented)
• Add relevant issue to release milestone
• Add pr to documentation list

UX Testing instructions

• Use the vpl database.
• Through the Specify Taxon Tree UI, merge two taxon nodes (e.g., 'Palaeoniscidae (in Palaeonisciformes)' into 'Palaeoniscidae (in Palaeoniscoidea)'). See that the taxon merge occurred without errors.

Running the merge:

Taxon Tree after merge:

• Try merging two other random taxon nodes in three to make sure no errors occur.

Development Testing instructions

• Query the database with:

  select count(*)
  from taxon t
  join taxon p on t.parentid = p.taxonid
  where t.nodenumber not between p.nodenumber and p.highestchildnodenumber;

• See that it returns a count of 0

• Run the renumber_tree('taxon') function directly via the Django Python shell.

  from specifyweb.backend.trees import extras
  from django.db import connection
  extras.renumber_tree('taxon')

  ❯ docker exec -it specify7 /bin/bash
  specify@4f497e2137b1:/opt/specify7$ ve/bin/python manage.py shell
  Python 3.12.3 (main, Aug 14 2025, 17:47:21) [GCC 13.3.0] on linux
  Type "help", "copyright", "credits" or "license" for more information.
  (InteractiveConsole)
  >>> from specifyweb.backend.trees import extras
  >>> from django.db import connection
  >>> extras.renumber_tree('taxon')
  [15/Sep/2025 12:14:47] [INFO] [specifyweb.specify.tree_extras:643] renumbering tree
  

• The command renumber_tree('taxon') ran without errors.

• Query the database with:

  select count(*)
  from taxon t
  join taxon p on t.parentid = p.taxonid
  where t.nodenumber not between p.nodenumber and p.highestchildnodenumber;

• The query should return 0.

• Through the Specify Taxon Tree UI, merge two taxon nodes (e.g., 'Palaeoniscidae (in Palaeonisciformes)' into 'Palaeoniscidae (in Palaeoniscoidea)'). See that the taxon merge occurred without errors.

• Go back to the python shell and run the validate_tree_numbering('taxon') command

from specifyweb.backend.trees import extras
from django.db import connection
extras.validate_tree_numbering('taxon')

• See that the validate_tree_numbering('taxon') command ran without errors

[bhumikaguptaa]

• Through the Specify Taxon Tree UI, merge two taxon nodes (e.g., 'Palaeoniscidae (in Palaeonisciformes)' into 'Palaeoniscidae (in Palaeoniscoidea)'). See that the taxon merge occurred without errors.

• Try merging two other random taxon nodes in three to make sure no errors occur.

'Palaeoniscidae' in the tree was not found to merge, otherwise i was able to perform random taxon merges.

[emenslin]

• Through the Specify Taxon Tree UI, merge two taxon nodes (e.g., 'Palaeoniscidae (in Palaeonisciformes)' into 'Palaeoniscidae (in Palaeoniscoidea)'). See that the taxon merge occurred without errors
• Try merging two other random taxon nodes in three to make sure no errors occur.

Could not recreate the issue in main but it seems to be working on this branch