funcs/func_cdr: Handle empty time values when extracting parsed values

funcs/func_cdr: Handle empty time values when extracting parsed values

When extracting timestamps that are parsed, time stamp values that are not set
(time values of 0.000000) should not actually result in a parsed string. The
value should be skipped, and the result of the CDR function should be an
empty string.

Prior to this patch, the result was fed to the time formatting, which would
result in an output of a date/time in 1969.

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/12@407747 65c4cc65-6c06-0410-ace0-fbb531ad65f3