Suppress separators for null fields in table formatters

[foozleface]

Fixes #6406
Contributed by @foozleface

When an object formatter has fields with a sep attribute and the related record is NULL, the separator still renders (e.g. a stray " - " with no value after it). The root cause is that blank_nulls wrapping around the nested formatter result prevents concat(sep, expr) from seeing the NULL, so the separator is unconditionally emitted.

Implementation

• In ObjectFormatter.objformat(), unwrap blank_nulls from the nested formatter result before assigning raw_expr, so that apply_stringish sees the raw expression and concat(sep, expr) correctly returns NULL when the nested formatter produced NULL. The outer blank_nulls still handles final display.
• Added a regression test that creates two AccessionAgent records (one with and one without an Accession) and verifies the separator only appears when the related record exists.

Testing instructions

• Create a formatter that references a relationship field with a sep attribute (e.g. AccessionAgent formatter with <field sep=" -Acc: " formatter="Accession">accession</field>)
• Create records where the relationship is NULL
• Verify the formatted output does not include the stray separator for NULL relationships
• Run python manage.py test specifyweb.backend.stored_queries.tests.test_format.test_format_query