fd: mmap: Don't use null span

We aren't allowed to use a null span with positive size, change the
internal interfaces to avoid it.

Change-Id: I6687d45a535c20b72f42a99e1ecdbd89f3796f99
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/test/pinned.cpp b/test/pinned.cpp
index 88c9bef..fc907fb 100644
--- a/test/pinned.cpp
+++ b/test/pinned.cpp
@@ -81,6 +81,7 @@
 TEST(PinnedRef, Fundamental)
 {
     Pinned<int> pi = 4;
+    // NOLINTNEXTLINE(clang-analyzer-core.UndefinedBinaryOperatorResult)
     EXPECT_EQ(4, PinnedRef<int>(pi));
     EXPECT_EQ(4, PinnedRef<const int>(pi));
     EXPECT_EQ(4, PinnedRef(pi));