Fixed #609: Don't work with empty patterns

autoload/vimtex/delim.vim

@@ -217,7 +217,7 @@ function! vimtex#delim#get_delim_regexes(...) " {{{1
   return exists('s:re')
         \ ? [s:re.delim_math.open, s:re.delim_math.close,
         \    s:re.delim_tex.open,  s:re.delim_tex.close]
-        \ : ['', '', '', '']
+        \ : []
 endfunction
 
 " }}}1

indent/tex.vim

@@ -107,6 +107,8 @@ let s:envs_enditem = s:envs_item . '\|' . s:envs_endlist
 
 " }}}1
 function! s:indent_delims(line, lnum, prev_line, prev_lnum) " {{{1
+  if empty(s:re_delims) | return 0 | endif
+
   let [l:text, l:math] = s:split(a:line, a:lnum)
   let [l:prev_tex, l:prev_math] = s:split(a:prev_line, a:prev_lnum, 'prev')