issue-468: Fix empty/whitespace string converting to array type

[gyurebalint-CID]

Fixes #468

Problem

When a configuration value is an empty or whitespace-only string and the target type is an array (e.g. string[]), StringArgumentValue.ConvertTo falls through to parsing logic that produces unexpected results instead of returning an empty array.

Changes

StringArgumentValue.cs

• Changed string.IsNullOrEmpty to string.IsNullOrWhiteSpace in the array-type guard, so that empty ("") and whitespace-only (" ") values both return an empty array.

StringArgumentValueTests.cs

• Added a [Theory] test with [InlineData("")] and [InlineData(" ")] asserting that empty/whitespace strings convert to an empty string[].
• Added a regression test asserting that a non-empty single string (e.g. "cookie1")
  does not silently convert to a single-element array, preserving the maintainer's
  original intent from issue InvalidCastException when enricher Args contains empty JSON array for params string[] parameter (regression in .NET 10) #468.

Why IsNullOrWhiteSpace over IsNullOrEmpty?

A whitespace-only string like " " is not meaningful array content. Without this, it would fall through and produce an array containing a whitespace string — a subtle bug. No one intentionally configures " " as valid array input; if structured values are needed, JSON arrays should be used instead.

[gyurebalint]

Hello everyone.
I think CI/CD needs approval for it to run.
If you have any comments, changes in mind, let me know and I'll adopt them.
Along with my other PR on a similar issue in this repo.
This is my private account, this PR was created with my work one.

[nblumhardt]

Thanks @gyurebalint-CID 👍