Only temporarily replace TMPDIR if it's defined to be an empty string

autoload/ale/util.vim

@@ -280,7 +280,7 @@ endfunction
 function! ale#util#Tempname() abort
     let l:clear_tempdir = 0
 
-    if has('unix') && empty($TMPDIR)
+    if exists('$TMPDIR') && empty($TMPDIR)
         let l:clear_tempdir = 1
         let $TMPDIR = '/tmp'
     endif
@@ -290,7 +290,6 @@ function! ale#util#Tempname() abort
     finally
         if l:clear_tempdir
             let $TMPDIR = ''
-            silent! unlet! $TMPDIR
         endif
     endtry
 

test/test_tmpdir_wrapper.vader

@@ -1,13 +1,32 @@
 Before:
-  Save $TMPDIR
+  let g:exists = exists('$TMPDIR')
+  let g:old_value = $TMPDIR
 
 After:
-  Restore
+  if g:exists
+    let $TMPDIR = g:old_value
+  else
+    silent! unlet! $TMPDIR
+  endif
 
-Execute(ale#util#Tempname should create files in /tmp if $TMPDIR isn't set):
+  unlet! g:exists
+  unlet! g:old_value
+
+Execute(ale#util#Tempname shouldn't set $TMPDIR to an empty string if it isn't set):
+  " You can't run this test twice on old Vim versions.
+  if has('unix')
+    Assert ale#util#Tempname() =~# '^/tmp'
+    Assert !exists('$TMPDIR'), '$TMPDIR exists where it shouldn''t'
+  endif
+
+Execute(ale#util#Tempname shouldn't replace $TMPDIR and reset them to an empty string.):
   if has('unix')
     let $TMPDIR = ''
     Assert ale#util#Tempname() =~# '^/tmp'
-    " We should unlet the environment variable again.
+
+    if !has('nvim')
+      Assert exists('$TMPDIR'), '$TMPDIR doesn''t exist where it should'
+    endif
+
     AssertEqual '', $TMPDIR
   endif