Handle value append mode in addProtoField

[kahlkevin]

• Wireshark Wiki LuaAPI/TreeItem page says of :add's text arguments:

Additional strings (or numbers) to be appended to the value text.
If multiple text arguments are provided, they are all concatenated
with a space delimiter to form a single string. Any text arguments
that are not strings or numbers are ignored (including nil)."

If text1 is a string type, all text arguments (including text1)
overwrite the full text of the TreeItem rather than appending to the
existing text. So, use nil for text1 to append the remaining
arguments to the value text. (By design? A bug turned feature?)"

• This commit fixes two problems:
  • nils should be ignored, but weren't being skipped
  • append vs overwrite when text1 is nil