metadata block drops date if missing author

[league]

I have documents with the old-style metadata blocks at the top:

% A Tale of Two Cities
% Charles Dickens
% 1859

but many leave the author field blank:

% A Tale of Two Cities
%
% 1859

Until recently I was using pandoc-1.9.4.5 and it would just leave out the author but still embed the date. Now, with pandoc-1.12.2.1 it will not propagate the date if the author is missing. Specifically, with pandoc -t json on the first example above produces:

[{"unMeta":{
  "author":{"t":"MetaList","c":
     [{"t":"MetaInlines","c":[{"t":"Str","c":"Charles"},{"t":"Space","c":[]},
      {"t":"Str","c":"Dickens"}]}]},
  "date":{"t":"MetaInlines","c":[{"t":"Str","c":"1859"}]},
  "title":{"t":"MetaInlines","c":[{"t":"Str","c":"A"},{"t":"Space","c":[]},
      {"t":"Str","c":"Tale"},{"t":"Space","c":[]},{"t":"Str","c":"of"},{"t":"Space","c":[]},
      {"t":"Str","c":"Two"},{"t":"Space","c":[]},{"t":"Str","c":"Cities"}]}}},[]]

and on the second example (with the missing author):

[{"unMeta":{
    "title":{"t":"MetaInlines","c":[{"t":"Str","c":"A"},{"t":"Space","c":[]},{"t":"Str","c":"Tale"},
       {"t":"Space","c":[]},{"t":"Str","c":"of"},{"t":"Space","c":[]},{"t":"Str","c":"Two"},
       {"t":"Space","c":[]},{"t":"Str","c":"Cities"}]}}},[]]

So the metadata block is missing the date, 1859.