[repo-assist] refactor: use isNull instead of <> null for BaseType checks (#538)

🤖 *This is an automated pull request from Repo Assist, an AI assistant.
Please review carefully before merging.*

## Summary

- Refactor: replace `ptdT.BaseType <> null` with `not (isNull
ptdT.BaseType)` at the two delegate-type-detection sites in the
generative assembly compiler (`src/ProvidedTypes.fs`).

## Rationale

`dotnet fsharplint` (rule [FL0065]((fsprojects.github.io/redacted)
flagged both occurrences: idiomatic F# prefers `isNull`/`not (isNull
...)` over structural inequality (`<> null`) against reference types,
since `<>` on some types can invoke a custom equality implementation
rather than a plain reference/null check. `isNull` is the intended,
purpose-built operator for this check.

## Trade-offs

- Purely cosmetic/idiomatic — no behavioral change. `BaseType` is a
`System.Type`, whose default equality is reference-based, so `<> null`
and `not (isNull ...)` are semantically identical here; the change only
improves clarity and lint-cleanliness.

## Test Status

- `dotnet build src/FSharp.TypeProviders.SDK.fsproj -c Release`:
**Passed** — 0 warnings, 0 errors (netstandard2.0 + net8.0)
- `dotnet test tests/FSharp.TypeProviders.SDK.Tests.fsproj -c Release`:
**Passed** — 165/165 tests, 0 failures, 0 skipped
- `dotnet fsharplint lint src/FSharp.TypeProviders.SDK.fsproj`: the two
FL0065 warnings are resolved; two unrelated pre-existing FL0072
(`failwith` argument uniqueness) warnings remain and are out of scope
for this change.

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Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: github-actions[bot]

src/ProvidedTypes.fs

@@ -15982,7 +15982,7 @@ namespace ProviderImplementation.ProvidedTypes
                 match ptdT with
                 | None -> ()
                 | Some ptdT ->
-                    let isDelegateType = ptdT.BaseType <> null && ptdT.BaseType.FullName = "System.MulticastDelegate"
+                    let isDelegateType = not (isNull ptdT.BaseType) && ptdT.BaseType.FullName = "System.MulticastDelegate"
                     for cinfo in ptdT.GetConstructors(bindAll) do
                         match cinfo with
                         | :? ProvidedConstructor as pcinfo when not (ctorMap.ContainsKey pcinfo)  ->
@@ -16065,7 +16065,7 @@ namespace ProviderImplementation.ProvidedTypes
 
                     // Delegate types (base = System.MulticastDelegate) use Runtime implementation; their
                     // constructor and Invoke/BeginInvoke/EndInvoke bodies are synthesised by the CLR.
-                    let isDelegateType = ptdT.BaseType <> null && ptdT.BaseType.FullName = "System.MulticastDelegate"
+                    let isDelegateType = not (isNull ptdT.BaseType) && ptdT.BaseType.FullName = "System.MulticastDelegate"
 
                     // Allow at most one constructor, and use its arguments as the fields of the type
                     let ctors =