fix: guard against blank FCM device tokens in sendMessageWithResult

[Copilot]

sendMessageWithResult only checked targetDeviceToken == null, allowing empty or whitespace-only tokens to pass through and trigger unnecessary Firebase calls and warn-level log noise.

Change

Replace the null check with !StringUtils.hasText(targetDeviceToken) to early-return false for null, empty, and whitespace tokens:

// Before
if (targetDeviceToken == null) {
    return false;
}

// After
if (!StringUtils.hasText(targetDeviceToken)) {
    return false;
}

Uses org.springframework.util.StringUtils, already present in the project.

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