fix(container): add to nil modifiers

[mdelapenya]

What does this PR do?

It sets the current modifier as the modifier if the original one was nil

Why is it important?

Avoid errors and simplify initialisation in client code

[Copilot]

Pull Request Overview

This PR modifies the WithAdditional*Modifier functions to handle nil original modifiers instead of returning an error. When the original modifier is nil, the new modifier is now directly applied rather than treating it as an error condition.

Key changes:

• Removed error returns when original modifiers are nil
• Added nil checks to apply the new modifier directly when no original exists
• Added test coverage for the nil original modifier scenarios

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
container/options.go	Updated three WithAdditional*Modifier functions to handle nil original modifiers by applying only the new modifier
container/options_test.go	Added "nil-original" test cases for each of the three modifier functions to verify the new behavior

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