Fix incorrect "NOT ... OR ..." pattern in LoadExclusions filter

[Copilot]

The pattern x is not null or "" evaluates as (x is not null) OR (x is ""), incorrectly including empty strings in the filter result. Replaced with !string.IsNullOrEmpty(x) to properly exclude both null and empty strings.

Before:

FrozenSet<string> excludeFiles = files.Where(x => x is not null or "").ToFrozenSet();

After:

FrozenSet<string> excludeFiles = files.Where(x => !string.IsNullOrEmpty(x)).ToFrozenSet();

Related: dotnet/roslyn#75506

To double check:

• The right tests are in and the right validation has happened. Guidance: https://github.com/dotnet/arcade/blob/main/Documentation/Validation.md

• Fixes Possible incorrect use of "NOT ... OR ..." pattern #16341

Original prompt

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This section details on the original issue you should resolve

<issue_title>Possible incorrect use of "NOT ... OR ..." pattern</issue_title>
<issue_description>Our internal tools are flagging these blocks of code as potentially improper use of the not ... or ... pattern matching syntax.

arcade/src/Microsoft.DotNet.Build.Tasks.Feed/src/PublishArtifactsInManifestBase.cs

Line 757 in 23d4316

FrozenSet<string> excludeFiles = files.Where(x => x is not null or "").ToFrozenSet();

See dotnet/roslyn#75506 for more information on why this pattern is being flagged. If this logic is correct, consider restructuring it to use clarifying () parentheses.</issue_description>

Comments on the Issue (you are @copilot in this section)

@mmitche Yeah, this is solidly wrong. Would be `x is not null and x is not ""`. Better to use `!string.IsNullOrEmpty(x)`.

• Fixes Possible incorrect use of "NOT ... OR ..." pattern #16341

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[mmitche]

@copilot remove this change.

[Copilot]

Done in 4d768ca