Fix segfault in declaration-export type walker on unregistered types

[ASDAlexander77]

Summary

• Fixes the segfault reported in Reproduce browser elements with this compiler #231: on Linux release builds (NDEBUG), the --export=all declaration-export walker in MLIRGenImpl.h dereferenced a null ClassInfo/InterfaceInfo/EnumInfo shared_ptr when a referenced type (e.g. one reachable only from a declare function signature in a .d.ts file) wasn't registered in the corresponding lookup map. The only guard was assert(...), which is compiled out in release builds, so the lookup miss fell straight through to a null-pointer dereference — matching the reporter's gdb backtrace through addClassDeclarationToExport.
• Replaces the unguarded asserts in addDependancyTypesToExportNoCheck and addTypeDeclarationToExportNoCheck with graceful early-returns, matching the existing safe null-check pattern already used elsewhere (MLIRTypeIterator.h) for the same kind of lookup.

Test plan

• Compiled MLIRGenClasses.cpp and MLIRGenModule.cpp (both include the modified header) — no new warnings/errors.
• Reporter's original BrowserLib repro project would be a good end-to-end check once the default lib with Uint8Array/typed-array support is available in this environment.

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