l10n: AGENTS.md: fix counter fallbacks

Reading a counter with "cmd || echo 0" breaks whenever the command
writes output but still exits non-zero. On zero matches, "grep -c"
prints 0 and exits with status 1, so the command substitution
captured two zeros and the arithmetic that follows failed with a
syntax error instead of evaluating to 0.

Fix by ignoring the exit status with "|| true" and defaulting the
empty value with "${VAR:-0}". Apply this to ENTRY_COUNT in both
l10n_one_batch() and review_one_batch(), and to the BATCH counter
read from po/review-batch.txt, which used the same fragile fallback.

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>

po/AGENTS.md

@@ -543,9 +543,9 @@ step 8 after step 6.
        DONE_PO="po/l10n-done.po"
        rm -f "$TODO_JSON" "$TODO_PO" "$DONE_JSON" "$DONE_PO"
 
-       ENTRY_COUNT=$(grep -c '^msgid ' "$PENDING" 2>/dev/null || echo 0)
+       ENTRY_COUNT=$(grep -c '^msgid ' "$PENDING" 2>/dev/null || true)
+       ENTRY_COUNT=${ENTRY_COUNT:-0}
        ENTRY_COUNT=$((ENTRY_COUNT > 0 ? ENTRY_COUNT - 1 : 0))
-
        if test "$ENTRY_COUNT" -gt $min_batch_size
        then
            if test "$ENTRY_COUNT" -gt $((min_batch_size * 8))
@@ -772,7 +772,8 @@ already present.
            cp "$INPUT_PO" "$PENDING"
        fi
 
-       ENTRY_COUNT=$(grep -c '^msgid ' "$PENDING" 2>/dev/null || echo 0)
+       ENTRY_COUNT=$(grep -c '^msgid ' "$PENDING" 2>/dev/null || true)
+       ENTRY_COUNT=${ENTRY_COUNT:-0}
        ENTRY_COUNT=$((ENTRY_COUNT > 0 ? ENTRY_COUNT - 1 : 0))
        if test "$ENTRY_COUNT" -eq 0
        then
@@ -796,7 +797,8 @@ already present.
            NUM=$ENTRY_COUNT
        fi
 
-       BATCH=$(cat "$BATCH_FILE" 2>/dev/null || echo 0)
+       BATCH=$(cat "$BATCH_FILE" 2>/dev/null || true)
+       BATCH=${BATCH:-0}
        BATCH=$((BATCH + 1))
        echo "$BATCH" >"$BATCH_FILE"