Join with some empty files

Prerequisites

• make sure you're are using the latest version by csvtk version
• read the usage

Describe your issue

• describe the problem
• provide a reproducible example

Input files:

$ echo -e "f,c1\n1,2" > 1.csv
$ echo -e "f,c2\n3,4" > 2.csv
$ echo -e "f,c3" > 3.csv

Two first files works fine:

$ csvtk join -f f --na NA --outer-join 1.csv 2.csv 
f,c1,c2
1,2,NA
3,NA,4

But when including the empty file:

$ csvtk join -f f --na NA --outer-join 1.csv 2.csv 3.csv 
[WARN] no data found in file: 3.csv
f,c1,c2
1,2,NA
3,NA,4

it does not show!
Any way that it shoes up, but just all NAs? For example:

f,c1,c2,c3
1,2,NA,NA
3,NA,4,NA

Start from the csvtk join command and reproduce the behavior with the three CSV files shown in the issue, including the empty-header-only file. Trace how input files with no data are handled and make the result retain that file's column with NA values, while preserving the existing outer-join output.