Exclude QuickFix from dectect#filetype function

[Nealium]

Fixes conflict specifically with vim-qf's QuickFix functions. QuickFix, in general, should be skipped with this function
Issue could effect other Plugins in similar situations, but this commit only solves QuickFix.

Specs:

• Vim: Vim 9 (64bit)
• OS: Windows 10 & Pop!_OS/Ubuntu

How to recreate

• Install both vim-qf & django-plug
• Goto any Django Project's view.py
• :copen + :cclose + :copen + :cclose
  • Open/Close 2 or more times

Result:

Error detected while processing FileType Autocommands for "*"..function <SNR>7_LoadFTPlugin

• Note: {x}_LoadFTPlugin x changes based on whatever pid vim{v}\ftplugin.vim is in :scriptnames
• Offending command in ftplugin.vim is: legacy exe b:undo_ftplugin

With tinkering, using :make, the error message [below] also shows up

Error detected while processig QuickFixCmdPost Autocommands for "make"..function qf#OpenQuickfix[17]..fileFileType Autocommands for "*"..function <SNR>6_LoadFTPlugin:
Line: 3:
E184: No such user-defined command: Filter

Theory:

# :help undo_ftplugin 
When the user does ":setfiletype xyz" the effect of the previous filetype
should be undone.

When setting the filetype of QuickFix to qf again, it undoes all vim-qf's functions which causes vim-qf to crash.
Seeing most Plugins are designed to define functions one time, Django-Plus should avoid inadvertently removing them; especially with QuickFix which is a special case

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Notes of behavior

function! djangoplus#detect#filetype(filename) abort

  " Passed with: djangoplus#detect#filetype(expand('<afile>:p')
  echom 'filename ' . a:filename

  " expanded inside the function
  echom 'expand path ' . expand('%:p')

  " filetype Output 
  echom 'filetype &l ' . &l:filetype

  " ... more stuff

endfunction

1) Opening views.py

filename            /path/to/project/app1/views.py
expand path         /path/to/project/app1/views.py
filetype &l         

2) Opening QuickFix

filename            /path/to/project/quickfix
expand path         
filetype &l         qf

Point:

This illustrates why the original empty(a:filename) isn't catching the QuickFix.

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Sorry for the Duplicate :(