[programs] Fix infinite loop when empty input is passed to trainer

When an empty input file was passed to the dictionary trainer, it would infinite loop.
The added test case exposes the bug, and is fixed with this PR.

tests/cli-tests/dict-builder/empty-input.sh

@@ -0,0 +1,9 @@
+#!/bin/sh
+set -e
+for i in $(seq 50); do
+    datagen -s$i > file$i
+done
+touch empty
+
+set -v
+zstd -q --train empty file*

tests/cli-tests/dict-builder/empty-input.sh.stderr.exact

@@ -0,0 +1 @@
+zstd -q --train empty file*