Fix brakeman handler when there is no output

ale_linters/ruby/brakeman.vim

@@ -5,6 +5,10 @@ let g:ale_ruby_brakeman_options =
 \   get(g:, 'ale_ruby_brakeman_options', '')
 
 function! ale_linters#ruby#brakeman#Handle(buffer, lines) abort
+    if len(a:lines) == 0
+        return []
+    endif
+
     let l:result = json_decode(join(a:lines, ''))
 
     let l:output = []

test/handler/test_brakeman_handler.vader

@@ -73,3 +73,9 @@ Execute(The brakeman handler should parse JSON correctly):
   \    ']',
   \  '}'
   \  ])
+
+Execute(The brakeman handler should parse JSON correctly when there is no output from brakeman):
+  AssertEqual
+  \ [],
+  \ ale_linters#ruby#brakeman#Handle(347, [
+  \ ])