slightly confusing JSON error context

These are some cases from the regression tests:

SELECT ''::json;                -- ERROR, no value
ERROR:  invalid input syntax for type json
LINE 1: SELECT ''::json;              ^
DETAIL:  The input string ended unexpectedly.
CONTEXT:  JSON data, line 1: 

SELECT '    '::json;            -- ERROR, no value
ERROR:  invalid input syntax for type json
LINE 1: SELECT '    '::json;              ^
DETAIL:  The input string ended unexpectedly.
CONTEXT:  JSON data, line 1:     

Perhaps we should either put some quotes around the whitespace at the
end of the CONTEXT lines, or write something like "end of input".
Otherwise this might look weird.  There might be more complicated cases
where it looks even weirder.