registration: Don't remove LDAP populated full name.
registration: Don't remove LDAP populated full name.

This commit fixes a bug that caused:
1: A valid full name on an onboarding form to be cleared after an
invalid submission.
2: Incorrectly cleared name populated from LDAP which was janky from
UX perspective.

Ideally we should disable name change for LDAP as next login
will overwrite any changes but I think that can be done in a
separate PR.

Fixes: #10867.

tommyip/zulip,shubhamdhama/zulip,synicalsyntax/zulip,rht/zulip,synicalsyntax/zulip,hackerkid/zulip,hackerkid/zulip,synicalsyntax/zulip,brainwane/zulip,synicalsyntax/zulip,kou/zulip,zulip/zulip,synicalsyntax/zulip,hackerkid/zulip,rht/zulip,brainwane/zulip,timabbott/zulip,rishig/zulip,timabbott/zulip,tommyip/zulip,zulip/zulip,rht/zulip,rht/zulip,rht/zulip,shubhamdhama/zulip,rishig/zulip,brainwane/zulip,showell/zulip,timabbott/zulip,showell/zulip,rht/zulip,tommyip/zulip,kou/zulip,andersk/zulip,andersk/zulip,showell/zulip,eeshangarg/zulip,andersk/zulip,tommyip/zulip,brainwane/zulip,rht/zulip,andersk/zulip,eeshangarg/zulip,rishig/zulip,kou/zulip,eeshangarg/zulip,zulip/zulip,timabbott/zulip,brainwane/zulip,zulip/zulip,punchagan/zulip,timabbott/zulip,hackerkid/zulip,tommyip/zulip,shubhamdhama/zulip,tommyip/zulip,tommyip/zulip,zulip/zulip,shubhamdhama/zulip,showell/zulip,timabbott/zulip,kou/zulip,eeshangarg/zulip,andersk/zulip,showell/zulip,hackerkid/zulip,kou/zulip,eeshangarg/zulip,punchagan/zulip,timabbott/zulip,synicalsyntax/zulip,andersk/zulip,rishig/zulip,punchagan/zulip,eeshangarg/zulip,andersk/zulip,shubhamdhama/zulip,hackerkid/zulip,showell/zulip,punchagan/zulip,brainwane/zulip,zulip/zulip,rishig/zulip,rishig/zulip,brainwane/zulip,shubhamdhama/zulip,synicalsyntax/zulip,shubhamdhama/zulip,kou/zulip,eeshangarg/zulip,punchagan/zulip,punchagan/zulip,zulip/zulip,hackerkid/zulip,showell/zulip,punchagan/zulip,kou/zulip,rishig/zulip