Add a missing = in BigInteger.cs

[LEI-Hongfaan]

-                if ((length == 0) || ((int)value[length - 1] > 0))
+                if ((length == 0) || ((int)value[length - 1] >= 0))
                 {
                     // We ne need to preserve the sign bit
                     length++;

[dotnet-policy-service]

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[tannergooding]

/backport to release/9.0-staging

[github-actions]

Started backporting to release/9.0-staging: https://github.com/dotnet/runtime/actions/runs/11802385408