Avoid IndexError with empty source dir

[jraby]

When running with an empty source directory, sigal would crash with the following error:

$ touch empty.py
$ mkdir emptydir
$ sigal build -c empty.py  emptydir/
Collecting albums, done.
  Sorting albums  [####################################]  100%
   Sorting media  [####################################]  100%
WARNING: No albums found.
Traceback (most recent call last):
  File "/home/jean/ws/src/sigal/env/bin/sigal", line 8, in <module>
    sys.exit(main())
             ^^^^^^
  File "/home/jean/ws/src/sigal/env/lib/python3.11/site-packages/click/core.py", line 1157, in __call__
    return self.main(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/jean/ws/src/sigal/env/lib/python3.11/site-packages/click/core.py", line 1078, in main
    rv = self.invoke(ctx)
         ^^^^^^^^^^^^^^^^
  File "/home/jean/ws/src/sigal/env/lib/python3.11/site-packages/click/core.py", line 1688, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/jean/ws/src/sigal/env/lib/python3.11/site-packages/click/core.py", line 1434, in invoke
    return ctx.invoke(self.callback, **ctx.params)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/jean/ws/src/sigal/env/lib/python3.11/site-packages/click/core.py", line 783, in invoke
    return __callback(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/jean/ws/src/sigal/src/sigal/__main__.py", line 215, in build
    stats_str += f"{format_stats(types[-1])}"
                                 ~~~~~^^^^
IndexError: list index out of range

This PR avoids printing stats when none have been gathered.

[codecov]

Codecov Report

Merging #498 (b8815bd) into main (c15d543) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #498   +/-   ##
=======================================
  Coverage   88.70%   88.70%           
=======================================
  Files          25       25           
  Lines        2062     2062           
=======================================
  Hits         1829     1829           
  Misses        233      233           

Files Changed	Coverage Δ
src/sigal/__main__.py	89.18% <100.00%> (ø)

[saimn]

Thanks!