Fix handling of null StringRef file buffers (#5428)

The current behavior hits UBSAN and ASAN issues.

Note, `RequiresNullTerminator` is already set to `false` in
`source_buffer.cpp`; setting it in `compile_helper.cpp` is making things
more consistent. The related logic is an [assert
fail](https://github.com/llvm/llvm-project/blob/main/llvm/lib/Support/MemoryBuffer.cpp#L52).

This was fuzzer-discovered.

toolchain/parse/tree_test.cpp

@@ -40,6 +40,11 @@ TEST_F(TreeTest, IsValid) {
   EXPECT_TRUE((*tree.postorder().begin()).has_value());
 }
 
+TEST_F(TreeTest, NullStringRef) {
+  Tree& tree = compile_helper_.GetTree(llvm::StringRef());
+  EXPECT_TRUE((*tree.postorder().begin()).has_value());
+}
+
 TEST_F(TreeTest, AsAndTryAs) {
   auto [tokens, tree_and_subtrees] =
       compile_helper_.GetTokenizedBufferWithTreeAndSubtrees("fn F();");