fix(WebViewClientDelegate): supply non-null favicon placeholder to delegates (#490)

[jim-daf]

Forward a non-null favicon to delegates to fix Kotlin crashes

Resolves #490 - IllegalArgumentException: Parameter specified as non-null is null: ... favicon.

Why Kotlin callers crash

WebView is allowed to deliver WebViewClient.onPageStarted with favicon == null
during early page-start (e.g. when the page hasn't yet announced an icon).
WebViewClientDelegate.onPageStarted simply forwards whatever it received to
the user's delegate.

Kotlin source compiles non-nullable parameter declarations into bytecode that
calls Intrinsics.checkParameterIsNotNull(...) on entry. A user delegate that
declares its onPageStarted(view: WebView?, url: String?, favicon: Bitmap) --
i.e. the favicon parameter is non-nullable -- therefore throws
IllegalArgumentException the moment AgentWeb forwards a null. This is the
exact stack trace in #490.

Fix

agentweb-core/src/main/java/com/just/agentweb/WebViewClientDelegate.java:

• In onPageStarted, when favicon is null, substitute a tiny lazily
  allocated 1×1 transparent ARGB_8888 placeholder bitmap before calling the
  delegate.
• Delegates with a Bitmap? signature observe no behavior change beyond
  receiving a non-null value.
• Delegates with a Bitmap (non-nullable) signature no longer crash.
• The placeholder is cached statically (double-checked) so the common path
  with a real favicon allocates nothing.

This avoids forcing every Kotlin caller to either remember to declare the
parameter as nullable or wire up a MiddlewareWebClient purely as a
null-shim (the workaround the issue author had to apply).

[Copilot]

Pull request overview

Fixes a Kotlin crash when WebView invokes onPageStarted with a null favicon by ensuring delegates always receive a non-null Bitmap.

Changes:

• Add a lazily-initialized 1×1 transparent Bitmap placeholder cached in WebViewClientDelegate.
• Substitute the placeholder for null favicons when forwarding onPageStarted to the delegate (and when calling super).

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[Copilot]

The placeholder bitmap is cached statically and the same instance is forwarded to external delegates. Because this instance escapes library ownership, a delegate could mutate or call recycle() on it, which would leave sFaviconPlaceholder in a bad state and potentially crash later calls. Consider either recreating the placeholder when placeholder.isRecycled() is true, or returning a fresh/copy placeholder per callback to avoid sharing a mutable/recyclable Bitmap instance with user code.

[Copilot]

The comment claims “nullable signatures see no observable change beyond receiving a non-null bitmap”, but receiving a non-null placeholder instead of null is an observable behavior change (callers may have been using null to mean “no favicon yet”). Please reword the comment to acknowledge this behavior change and/or describe how callers can detect the placeholder if needed.

Suggested change

	// signatures keep working; nullable signatures see no observable
	// change beyond receiving a non-null bitmap.
	// signatures keep working. This is an observable change for nullable
	// consumers because they receive the shared placeholder instead of
	// null; callers that need to detect "no favicon yet" can compare the
	// received Bitmap by identity with faviconPlaceholder().

[Justson]

感谢贡献，但这个修法方向上有问题，暂不合入。

PR 在 favicon 为 null 时塞入一个静态共享的 1×1 透明 Bitmap。这会带来几个副作用：

1. 违反平台契约
WebViewClient#onPageStarted 的 favicon 本身就是可空的，这是 framework 的既定约定。下游普遍存在 if (favicon != null) { showIcon(favicon) } 这类写法，改动后它们会永远拿到一张假图，把「不显示图标」变成「显示一个 1×1 透明块」。

2. 静态共享 Bitmap 有回收风险
sFaviconPlaceholder 是全局单例并直接交给外部回调。任何使用者认为自己持有该 Bitmap 而调用 recycle()，后续复用就会抛 Cannot draw a recycled Bitmap / IllegalStateException，且这类崩溃极难定位。

3. 问题根因不在库这一侧
issue #490 的崩溃源于 Kotlin 使用方把 override 签名写成了非空 Bitmap：

override fun onPageStarted(view: WebView?, url: String?, favicon: Bitmap) { }  // ❌
override fun onPageStarted(view: WebView?, url: String?, favicon: Bitmap?) { } // ✅

正确写法是把参数声明为可空 Bitmap?。在库里伪造数据来迎合调用方的错误签名，属于用一个更隐蔽的问题去掩盖一个明确的问题。

AgentWeb 自身在 DefaultWebClient#onPageStarted 中只是透传 favicon、从不解引用，所以库这一侧没有需要修复的缺陷。issue #490 我会补充说明后按「使用方签名问题」处理。

另外，本 PR 与同批次的 #1086 是对同一 issue 的两种互相矛盾的方案（一个主张「就该可空」、一个主张「要填非空」），也侧面说明这个方向需要再斟酌。