fix: Server error from api_get_session_materials()

[nectostr]

Fixes #5877

Guard with None added.
I checked the frontend reaction and it seems like JS will just create empty url instance that won’t be shown as URL. However I did no create test for that and I won’t be able to, so maybe further actions needed (maybe not)

[codecov]

Codecov Report

Merging #6025 (54ba9f6) into main (09f3477) will decrease coverage by 0.04%.
The diff coverage is 89.47%.

@@            Coverage Diff             @@
##             main    #6025      +/-   ##
==========================================
- Coverage   88.67%   88.64%   -0.04%     
==========================================
  Files         288      288              
  Lines       40001    40006       +5     
==========================================
- Hits        35471    35463       -8     
- Misses       4530     4543      +13     

Impacted Files	Coverage Δ
ietf/doc/views_stats.py	74.80% <ø> (ø)
ietf/group/urls.py	100.00% <ø> (ø)
ietf/group/views.py	90.61% <ø> (+0.08%)	⬆️
ietf/ietfauth/widgets.py	84.61% <ø> (ø)
ietf/submit/views.py	84.60% <ø> (ø)
ietf/nomcom/views.py	92.90% <33.33%> (-0.22%)	⬇️
ietf/doc/utils.py	87.15% <100.00%> (+0.01%)	⬆️
ietf/doc/views_status_change.py	93.38% <100.00%> (ø)
ietf/meeting/views.py	91.03% <100.00%> (+<0.01%)	⬆️
ietf/nomcom/templatetags/nomcom_tags.py	70.83% <100.00%> (+3.64%)	⬆️
... and 2 more

... and 4 files with indirect coverage changes