[tests] Guard prefetch GUID optional access for clang-tidy

The block-prefetch GUID test dereferenced the TryParse result after a Catch2
REQUIRE, which clang-tidy's bugprone-unchecked-optional-access does not track as
a guard (-warnings-as-errors). Add an explicit `if (has_value())` so the optional
access is statically checked while keeping the REQUIRE as the failure signal.

Signed-off-by: Christian Parpart <c.parpart@lastrada.net>

Author: Christian Parpart

src/tests/SqlStatementPrefetchTests.cpp

@@ -514,7 +514,8 @@ TEST_CASE_METHOD(SqlTestFixture, "Prefetch: GUID column round-trips", "[prefetch
     {
         auto const parsedUid = SqlGuid::TryParse(std::format("12345678-1234-1234-1234-{:012}", i));
         REQUIRE(parsedUid.has_value());
-        (void) stmt.Execute(static_cast<std::int64_t>(i), *parsedUid);
+        if (parsedUid.has_value()) // explicit guard so the optional access is statically checked
+            (void) stmt.Execute(static_cast<std::int64_t>(i), *parsedUid);
     }
 
     auto readGuids = [&](SqlStatement& source) {