SystemBars: safe area CSS injection throws "Cannot read properties of null" on startup (Android 16)

### Capacitor Version

Hit this while upgrading to Capacitor 8 and targeting SDK 36. Every app launch logs:

```
E Capacitor/Console: File: - Line 6 - Msg: Error injecting safe area CSS: TypeError: Cannot read properties of null (reading 'style')
```

Environment: @capacitor/android 8.4.1, compileSdk/targetSdk 36, AGP 8.13.0, Gradle 8.14.3, JDK 21, Samsung device on Android 16.

## What's happening

It's coming from `SystemBars.injectSafeAreaCSS()` (~line 250 in 8.4.1). The injected script does:

```java
"""
try {
document.documentElement.style.setProperty("--safe-area-inset-top", "%dpx");
document.documentElement.style.setProperty("--safe-area-inset-right", "%dpx");
document.documentElement.style.setProperty("--safe-area-inset-bottom", "%dpx");
document.documentElement.style.setProperty("--safe-area-inset-left", "%dpx");
} catch(e) { console.error('Error injecting safe area CSS:', e); }
"""
```

`document.documentElement` is null when this runs. The "Line 6" in the logcat output is the catch line inside that text block, which lines up.

As far as I can tell, `injectSafeAreaCSS` gets called from the `setOnApplyWindowInsetsListener` callback, and that fires before the WebView document exists on first layout. The `bridge.getWebView() != null` guard passes — the WebView object is there — but there's no document in it yet. Later passes work fine, which is why the CSS variables do end up set correctly and nothing visibly breaks.

## Why I couldn't just work around it

Tried `insetsHandling: "disable"` first, which was a mistake — on Android 16 SystemBars is the only thing actually insetting content below the status bar. `StatusBar.setOverlaysWebView()` uses `setSystemUiVisibility`, which is a no-op at API 35+. So disabling the plugin to kill the error breaks the layout the plugin is there to provide. Ended up patching the framework instead, which isn't great to have to carry.

## Fix

Null guard in the injected script:

```js
try {
if (document.documentElement) {
document.documentElement.style.setProperty("--safe-area-inset-top", "%dpx");
// ...etc
}
} catch(e) { console.error('Error injecting safe area CSS:', e); }
```

No-op when the DOM's already there, so no behaviour change. Ran this on a physical Android 16 device — error's gone, layout is identical, no other console errors.

## Repro

Cap 8.4.1 Android app, targetSdk 36, `viewport-fit=cover` in the viewport meta, launch on an Android 16 device, watch the console.

Happy to put up a PR if it's wanted.

### Other API Details

```Shell

```

### Platforms Affected

- [ ] iOS
- [x] Android
- [ ] Web

### Current Behavior

On every app launch, an error is logged to the console:

Error injecting safe area CSS: TypeError: Cannot read properties of null (reading 'style')

Full logcat line:
E Capacitor/Console: File: - Line 6 - Msg: Error injecting safe area CSS: TypeError: Cannot read properties of null (reading 'style')

It comes from SystemBars.injectSafeAreaCSS(). The injected script calls
document.documentElement.style.setProperty(...), but document.documentElement is
null at that moment — the "Line 6" in the log is the catch line inside that Java
text block, which lines up with the setProperty calls throwing.

As far as I can tell, injectSafeAreaCSS gets called from the
setOnApplyWindowInsetsListener callback, and that fires before the WebView
document exists on first layout. The `bridge.getWebView() != null` guard passes
— the WebView object is there — but there's no document in it yet.

Later passes succeed, so the --safe-area-inset-* variables do end up set
correctly and nothing visibly breaks. It's just an error on every startup.

Worth noting: insetsHandling: "disable" isn't a viable workaround. On Android 16
SystemBars is the only thing actually insetting content below the status bar —
StatusBar.setOverlaysWebView() uses setSystemUiVisibility, which is a no-op at
API 35+. So disabling the plugin to silence the error breaks the layout the
plugin is there to provide.

### Expected Behavior

The injected script should no-op when the document isn't available yet, rather
than throwing and logging an error on every launch.

A null guard is enough:

try {
if (document.documentElement) {
document.documentElement.style.setProperty("--safe-area-inset-top", "%dpx");
...
}
} catch(e) { console.error('Error injecting safe area CSS:', e); }

It's a no-op whenever the DOM already exists, so no behaviour change. I applied
exactly this locally and verified on a physical Android 16 device — the error is
gone, no other console errors, and the layout is unchanged.

Happy to open a PR if that's useful.

### Project Reproduction

No standalone repro repo — but this should reproduce on any Capacitor 8.4.1 Android app targeting SDK 36 with viewport-fit=cover in the viewport meta tag. Launch on an Android 16 device and watch the console on startup. The throw is inside Capacitor's own injected script, so I don't think app code is involved.

### Additional Information

_No response_

Start at SystemBars.injectSafeAreaCSS(), around line 250 in Capacitor 8.4.1, and inspect its setOnApplyWindowInsetsListener caller and injected script. Reproduce with an Android app targeting SDK 36 on a physical Android 16 device; done means startup emits no safe-area CSS error and the layout and CSS variables remain unchanged.

Written by the indexing model from the issue text.