Test FPDFPageObj_SetIsActive() with nullptr input

Provide better test coverage for this new API.

Change-Id: Ib9cd1bd23f7059568a163c6f4b75fb5e460f45fa
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/126431
Reviewed-by: Thomas Sepez <tsepez@google.com>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
diff --git a/fpdfsdk/fpdf_editpage_embeddertest.cpp b/fpdfsdk/fpdf_editpage_embeddertest.cpp
index 54e2f59..b578550 100644
--- a/fpdfsdk/fpdf_editpage_embeddertest.cpp
+++ b/fpdfsdk/fpdf_editpage_embeddertest.cpp
@@ -576,6 +576,10 @@
     CloseSavedPage(saved_page);
     CloseSavedDocument();
   }
+
+  // Negative testing.
+  EXPECT_FALSE(FPDFPageObj_SetIsActive(nullptr, false));
+  EXPECT_FALSE(FPDFPageObj_SetIsActive(nullptr, true));
 }
 
 TEST_F(FPDFEditPageEmbedderTest, Bug378120423) {