🍒 pick e224fd6: 🥅 Validate that Atom and Flag are not empty

`Atom` and `Flag` have only been used for argument validation since
v0.6.4 (as well as v0.5.14 and v0.4.24), and they validated for absense
of `atom-specials`.  But they failed to check that the strings are not
empty.

While this could be used to create syntax errors, I don't believe it
amounts a security vulnerability.  The result would be no different from
any other `BAD` server response, which an application must be prepared
to handle.

Author: nevans

lib/net/imap/command_data.rb

@@ -241,6 +241,8 @@ def validate
           or raise DataFormatError, "#{self.class} must be ASCII only"
         data.match?(ResponseParser::Patterns::ATOM_SPECIALS) \
           and raise DataFormatError, "#{self.class} must not contain atom-specials"
+        data.empty? \
+          and raise DataFormatError, "#{self.class} must not be empty"
       end
 
       def send_data(imap, tag)

test/net/imap/test_command_data.rb

@@ -85,6 +85,7 @@ def send_data(*data, tag: TAG)
       "with_quoted_specials\\",
       "with\rCR",
       "with\nLF",
+      "", # empty
     ].each do |symbol|
       assert_raise_with_message(Net::IMAP::DataFormatError, /\batom\b/i) do
         imap.send_data Atom[symbol]
@@ -116,6 +117,7 @@ def send_data(*data, tag: TAG)
       :"with_quoted_specials\\",
       :"with\rCR",
       :"with\nLF",
+      :"", # empty
     ].each do |symbol|
       assert_raise_with_message(Net::IMAP::DataFormatError, /\bflag\b/i) do
         imap.send_data Flag[symbol]