Fix crash on long-press when a placeholder is the only popup key

[MiMoHo]

Fixes #1324.

Problem

When a key has a placeholder popup as its only popup key, and the popup key order is set so that only "Layout" popups are used, long-pressing the key crashes the keyboard.

Reproduction (from #1324):

{ "label": "ক", "popup": { "relevant": [ { "type": "placeholder" } ] } }

1. Use a layout with such a key.
2. Settings → set popup key order to "Layout" only.
3. Long-press the key.

Cause

createPopupKeysArray produces a non-empty array for the placeholder, but the placeholder resolves to an empty string. After PopupKeySpec.filterOutEmptyString(...) removes it, the result is an empty (but non-null) array. Key.KeyParams then still sets ACTION_FLAGS_ENABLE_LONG_PRESS, so a long-press opens a popup keyboard with zero keys, which throws in PopupKeysKeyboard.Builder (division by / layout for zero columns).

Fix

Only enable long-press / build mPopupKeys when the filtered array is actually non-empty:

if (finalPopupKeys != null && finalPopupKeys.length > 0) {

This makes a placeholder-only popup set behave like "no popups", matching the expected behaviour described in the issue (long-press does nothing instead of crashing).

Tests

Added two tests to ParserTest:

• placeholderAsOnlyPopup – a placeholder-only popup set yields mPopupKeys == null and long-press disabled (this crashed before the fix).
• placeholderNextToOtherPopup – a placeholder alongside a real popup key keeps only the real key.

All unit tests pass (testRunTestsUnitTest).