Handle empty chat output better

If the output was empty we would end up running `''.split("\n")`,
producing an empty Array. Slicing an empty Array using a range such as
1..-1 would then produce a nil value, instead of an empty Array. This in
turn would cause a NoMethodError since "join" is not defined for
NilClass.

ee/lib/gitlab/chat/output.rb

@@ -59,7 +59,18 @@ module Gitlab
       #
       # output - A `String` containing the output of a trace section.
       def without_executed_command_line(output)
-        output.split("\n")[1..-1].join("\n")
+        # If `output.split("\n")` produces an empty Array then the slicing that
+        # follows it will produce a nil. For example:
+        #
+        #     "\n".split("\n")        # => []
+        #     "\n".split("\n")[1..-1] # => nil
+        #
+        # To work around this we only "join" if we're given an Array.
+        if (converted = output.split("\n")[1..-1])
+          converted.join("\n")
+        else
+          ''
+        end
       end
 
       # Returns the trace section for the given name, or `nil` if the section

ee/spec/lib/gitlab/chat/output_spec.rb

@@ -71,6 +71,14 @@ describe Gitlab::Chat::Output do
       expect(output.without_executed_command_line("hello\nworld"))
         .to eq('world')
     end
+
+    it 'returns an empty String when the input is empty' do
+      expect(output.without_executed_command_line('')).to eq('')
+    end
+
+    it 'returns an empty String when the input consits of a single newline' do
+      expect(output.without_executed_command_line("\n")).to eq('')
+    end
   end
 
   describe '#find_build_trace_section' do