communicators/ssh: more nil guards [GH-6225]

CHANGELOG.md

@@ -50,6 +50,7 @@ BUG FIXES:
   - core: use the correct private key when packaging a box [GH-6406]
   - commands/rdp: prefer `xfreerdp` if it is available on Linux [GH-6475]
   - commands/up: the `--provision-with` flag works with provisioner names [GH-5981]
+  - communicator/ssh: fix potential crash case with PTY [GH-6225]
   - communicator/winrm: respect `boot_timeout` setting [GH-6229]
   - communicator/winrm: execute scheduled tasks immediately on Windows XP
       since elevation isn't required [GH-6195]

plugins/communicators/ssh/communicator.rb

@@ -611,6 +611,7 @@ module VagrantPlugins
           end
 
           data = pty_stdout[/.*#{PTY_DELIM_START}(.*?)#{PTY_DELIM_END}/m, 1]
+          data ||= ""
           @logger.debug("PTY stdout parsed: #{data}")
           yield :stdout, data if block_given?
         end