Remove unnecessary exe check for black

autoload/ale/fixers/black.vim

@@ -18,10 +18,6 @@ endfunction
 function! ale#fixers#black#Fix(buffer) abort
     let l:executable = ale#fixers#black#GetExecutable(a:buffer)
 
-    if !executable(l:executable)
-        return 0
-    endif
-
     let l:exec_args = l:executable =~? 'pipenv$'
     \   ? ' run black'
     \   : ''
@@ -29,7 +25,7 @@ function! ale#fixers#black#Fix(buffer) abort
     let l:options = ale#Var(a:buffer, 'python_black_options')
 
     return {
-    \   'command': ale#Escape(l:executable) . l:exec_args
+    \   'command': ale#Escape(l:executable. l:exec_args)
     \       . (!empty(l:options) ? ' ' . l:options : '')
     \       . ' -',
     \}

test/fixers/test_black_fixer_callback.vader

@@ -5,6 +5,7 @@ Before:
   " Use an invalid global executable, so we don't match it.
   let g:ale_python_black_executable = 'xxxinvalid'
   let g:ale_python_black_options = ''
+  let g:ale_python_black_auto_pipenv = 0
 
   call ale#test#SetDirectory('/testplugin/test/fixers')
   silent cd ..
@@ -21,10 +22,6 @@ After:
   call ale#test#RestoreDirectory()
 
 Execute(The black callback should return the correct default values):
-  AssertEqual
-  \ 0,
-  \ ale#fixers#black#Fix(bufnr(''))
-
   silent execute 'file ' . fnameescape(g:dir . '/python_paths/with_virtualenv/subdir/foo/bar.py')
   AssertEqual
   \ {'command': ale#Escape(ale#path#Simplify(g:dir . '/python_paths/with_virtualenv/env/' . b:bin_dir . '/black')) . ' -'},
@@ -40,7 +37,8 @@ Execute(The black callback should include options):
 
 Execute(Pipenv is detected when python_black_auto_pipenv is set):
   let g:ale_python_black_auto_pipenv = 1
+  call ale#test#SetFilename('/testplugin/test/python_fixtures/pipenv/whatever.py')
 
   AssertEqual 
-  \ {'command': 'pipenv run mypy' },
+  \ {'command': ale#Escape('pipenv run black') . ' -'},
   \ ale#fixers#black#Fix(bufnr(''))