Make sure, the highlighting group will be defined

If a color value of ['', '', 'NONE', 'NONE', ''] is given as value to
the highlighting group, the resulting group definition would look like
this:

hi Normal ctermfg=NONE ctermbg=NONE

which would result in the highlighting group being cleared (or even no
set at all), therefore check that at least one other value exists and if
not fall back to the highlighting definition of the Normal group.

autoload/airline/highlighter.vim

@@ -72,6 +72,7 @@ function! airline#highlighter#exec(group, colors)
   if len(colors) == 4
     call add(colors, '')
   endif
+  let colors = s:CheckDefined(colors)
   if old_hi != colors
     let cmd = printf('hi %s %s %s %s %s %s %s %s',
         \ a:group, s:Get(colors, 0, 'guifg=', ''), s:Get(colors, 1, 'guibg=', ''),
@@ -82,6 +83,24 @@ function! airline#highlighter#exec(group, colors)
   endif
 endfunction
 
+function! s:CheckDefined(colors)
+  " Checks, whether the definition of the colors is valid and is not empty or NONE
+  " e.g. if the colors would expand to this:
+  " hi airline_c ctermfg=NONE ctermbg=NONE
+  " that means to clear that highlighting group, therefore, add a special term=NONE
+  " argument
+  for val in a:colors
+    if !empty(val) && val !=# 'NONE'
+      return a:colors
+    endif
+  endfor
+  " this adds the bold attribute to the term argument of the :hi command,
+  " but at least this makes sure, the group will be defined
+  let fg = synIDattr(synIDtrans(hlID('Normal')), 'fg', 'cterm')
+  let bg = synIDattr(synIDtrans(hlID('Normal')), 'bg', 'cterm')
+  return a:colors[0:1] + [fg, bg] + [a:colors[4]]
+endfunction
+
 function! s:Get(dict, key, prefix, default)
   if get(a:dict, a:key, a:default) isnot# a:default
     return a:prefix. get(a:dict, a:key)