🍒 [Swiftify] don't unwrap pointers for UP! when not needed

[hnrklssn]

• Explanation:
  When passing an empty Span to the safe wrapper of a function annotated with counted_by without _Nullable the wrapper would unwrap a nil base pointer, even though the function accepts a nil pointer (and counted_by promises not to dereference it, since the length is 0). This updates the generated wrapper to not unwrap when not necessary.
• Scope:
  Can now pass empty Span to counted_by without explicit nullability (but still not _Nonnull, since that signature explicitly bans null) without trapping. No signature change.
• Issues:
  rdar://177555277
• Original PRs:
  [Swiftify] don't unwrap pointers for UP! when not needed #89455
• Risk:
  Low, should strictly allow more cases before (at runtime)
• Testing:
  Lit testing
• Reviewers:
  @j-hui

[hnrklssn]

stacked on top of #89732

[hnrklssn]

@swift-ci please test