split-files: fail early if we split-files produces an empty output

We would fail later anyway, because rpm refuses %files with an empty filelist
file. But this is much later, after %check, so let's fail already in %install.

[skip changelog]

split-files.py

@@ -249,3 +249,8 @@ for file in files(buildroot):
     suffix = '*' if '/man/' in n else ''
 
     print(f'{prefix}{n}{suffix}', file=o)
+
+if [print(f'ERROR: no file names were written to {o.name}')
+    for o in outputs.values()
+    if o.tell() == 0]:
+    sys.exit(1)