SSave: check for empty input

autoload/startify.vim

@@ -155,10 +155,16 @@ function! startify#session_save(...) abort
       return
     endif
   endif
-  let spath = s:session_dir . startify#get_separator() . (exists('a:1')
-        \ ? a:1
-        \ : input('Save under this session name: ', fnamemodify(v:this_session, ':t'), 'custom,startify#session_list_as_string'))
-        \ | redraw
+  if exists('a:1')
+    let spath = s:session_dir . startify#get_separator() . a:1
+  else
+    let i = input('Save under this session name: ', fnamemodify(v:this_session, ':t'), 'custom,startify#session_list_as_string')
+    redraw
+    if empty(i)
+      echo 'You gave an invalid name!'
+      return
+    endif
+  endif
   let spath = s:escape(spath)
   if !filereadable(spath)
     execute 'mksession '. spath | echo 'Session saved under: '. spath