Allow empty keys to clear IRK/EncKey

This commit modifies the handling of Identity Resolving Key (IRK) and Encryption Key (EncKey) to allow users to clear these values by providing an empty input.

Specifically:
- In `GeneralSettingsFragment.kt`, the `onKey` callbacks for both IRK and EncKey dialogs now use `takeIf { it.isNotEmpty() }` after converting the input hex string to a byte array. This ensures that an empty input results in `null` being set for the respective key.
- In `PodMonitor.kt`, when determining the main device, `mainDeviceIdentityKey.value` is now checked with `takeIf { it.isNotEmpty() }` to ensure an empty IRK is treated as no IRK being configured.
- In `AppleFactory.kt`, when attempting to decrypt the private payload, `mainDeviceEncryptionKey.value` is now checked with `takeIf { it.isNotEmpty() }` to ensure an empty EncKey prevents decryption attempts.

app/src/main/java/eu/darken/capod/main/ui/settings/general/GeneralSettingsFragment.kt

@@ -55,7 +55,9 @@ class GeneralSettingsFragment : PreferenceFragment3() {
             AirPodKeyInputDialog(requireContext()).create(
                 mode = AirPodKeyInputDialog.Mode.IRK,
                 current = generalSettings.mainDeviceIdentityKey.value?.toHex() ?: "",
-                onKey = { generalSettings.mainDeviceIdentityKey.value = it.fromHex() },
+                onKey = { raw ->
+                    generalSettings.mainDeviceIdentityKey.value = raw.fromHex().takeIf { it.isNotEmpty() }
+                },
                 onGuide = { webpageTool.open("https://github.com/d4rken-org/capod/wiki/airpod-Keys") }
             ).show()
             true
@@ -64,7 +66,9 @@ class GeneralSettingsFragment : PreferenceFragment3() {
             AirPodKeyInputDialog(requireContext()).create(
                 mode = AirPodKeyInputDialog.Mode.ENC,
                 current = generalSettings.mainDeviceEncryptionKey.value?.toHex() ?: "",
-                onKey = { generalSettings.mainDeviceEncryptionKey.value = it.fromHex() },
+                onKey = { raw ->
+                    generalSettings.mainDeviceEncryptionKey.value = raw.fromHex().takeIf { it.isNotEmpty() }
+                },
                 onGuide = { webpageTool.open("https://github.com/d4rken-org/capod/wiki/airpod-Keys") }
             ).show()
             true