BpfRingBuf.h - implement isEmpty()

Test: TreeHugger, atest BpfRingbufTest
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I23020869ab665c029c3219b1b39c3749be6e6992
diff --git a/staticlibs/native/bpf_headers/BpfRingbufTest.cpp b/staticlibs/native/bpf_headers/BpfRingbufTest.cpp
index 6c0841c..e4de812 100644
--- a/staticlibs/native/bpf_headers/BpfRingbufTest.cpp
+++ b/staticlibs/native/bpf_headers/BpfRingbufTest.cpp
@@ -72,12 +72,15 @@
 
     auto result = BpfRingbuf<uint64_t>::Create(mRingbufPath.c_str());
     ASSERT_RESULT_OK(result);
+    EXPECT_TRUE(result.value()->isEmpty());
 
     for (int i = 0; i < n; i++) {
       RunProgram();
     }
 
+    EXPECT_FALSE(result.value()->isEmpty());
     EXPECT_THAT(result.value()->ConsumeAll(callback), HasValue(n));
+    EXPECT_TRUE(result.value()->isEmpty());
     EXPECT_EQ(output, TEST_RINGBUF_MAGIC_NUM);
     EXPECT_EQ(run_count, n);
   }