fix: null check for common.ts/assign

[pmingkr]

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Description

if the dataSource failed to response on sorting, it's errored on firing the errorResponse event.
eventBus.ts/createEventBus/trigger method uses setInstance method with the tui.Grid instance.
but the tui.Grid instance contains null at grid.store.data.filters.
and eventually it calls common.ts/assign with null value.
So I added null-check for it on this PR.

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