Fix PydocstringFormat deletes file contents if docstrings already present

autoload/pydocstring.vim

@@ -76,7 +76,12 @@ endfunction
 
 function! s:exit_callback(msg) abort
   unlet s:job " Needed for Neovim
-  if len(s:results)
+  let length = len(s:results)
+  if length
+    if length == 1 && s:results[0] == ''
+      let s:results = []
+      return
+    endif
     let view = winsaveview()
     silent execute '% delete'
     call setline(1, s:results)
@@ -94,6 +99,7 @@ function! s:execute(cmd, lines, indent, start_lineno, cb, ex_cb) abort
     return
   endif
 
+  let s:results = []
   if has('nvim')
     if exists('s:job')
       call jobstop(s:job)

tests/format.vader

@@ -39,3 +39,40 @@ Expect python:
             :param arg2:
             """
             pass
+
+
+Given python (Format all twice):
+    class Foo:
+        def foo(arg1):
+            pass
+
+
+        def bar(arg1, arg2):
+            pass
+
+
+Execute:
+    :PydocstringFormat
+    :sleep 1
+    :PydocstringFormat
+    :sleep 1
+
+Expect python:
+    class Foo:
+        """Foo."""
+
+        def foo(arg1):
+            """foo.
+
+            :param arg1:
+            """
+            pass
+
+
+        def bar(arg1, arg2):
+            """bar.
+
+            :param arg1:
+            :param arg2:
+            """
+            pass