fill 0-row data.frame with 1 line of NAs

Why does one want to replace a zero-row data.frame
with a one-row data.frame of NA's?  Unless this is for
an external program that cannot handle zero-row inputs,
this suggests that there is an unnecessary limitation (i.e.,
a bug) in the R code that uses this data.frame.

I'm running an apply(df, 1, f) function, where f() matches a df$string
in another matrix and fetches data associated with this string. When
no match is made I do not need a zero-row data frame, but to preserve
the structure of the original df I need a data frame with 1 row of
NAs. There may be a nicer approach, but I'm not aware of any.

Regards
Liviu