Fix Null type in FlutterLoadAdError domain for adsNextGenSdk

[LTPhantom]

Description

Added a null check on responseInfo.getMediationAdapterClassName(). If null, domain falls back to "com.google.android.libraries.ads.mobile.sdk", matching the fallback behavior when error.getResponseInfo() is null.

Related Issues

• Resolves [Next-gen SDK] LoadAdError decoding throws "type 'Null' is not a subtype of type 'String'" when mediationAdapterClassName is nul #1460

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[malandr2]

LGTM for DevRel