Optimize empty string compares

[dawedawe]

Description

According to CA1820 it's more performant to do checks of empty strings by using String.IsNullOrEmpty() or the Length property.

Checklist

• Test cases added

• Performance benchmarks added in case of performance changes

• Release notes entry updated:

  Please make sure to add an entry with short succint description of the change as well as link to this pull request to the respective release notes file, if applicable.

  Release notes files:

  • If anything under src/Compiler has been changed, please make sure to make an entry in docs/release-notes/FSharp.Compiler.Service/<version>.md, where <version> is usually "highest" one, e.g. 42.8.200
  • If language feature was added (i.e. LanguageFeatures.fsi was changed), please add it to docs/releae-notes/Language/preview.md
  • If a change to FSharp.Core was made, please make sure to edit docs/release-notes/FSharp.Core/<version>.md where version is "highest" one, e.g. 8.0.200.

  Examples of release notes entries:

  • Respect line limit in quick info popup - Respect line limit in QuickInfo #16208
  • More inlines for Result module - Inline results module #16106
  • Miscellaneous fixes to parens analysis - Parens: miscellaneous fixes #16262

  If you believe that release notes are not necessary for this PR, please add NO_RELEASE_NOTES label to the pull request.

[psfinaki]

Thanks!