Migrate from android::String isEmpty to empty

This empty method is different from the old one - it aligns with
std::string definition.

Bug: 295394788
Test: make checkbuild
Change-Id: I890b3dfa930002f4406e9d9aec024f94ada1cad0
diff --git a/media/libnblog/Reader.cpp b/media/libnblog/Reader.cpp
index 36e4da5..26d12cd 100644
--- a/media/libnblog/Reader.cpp
+++ b/media/libnblog/Reader.cpp
@@ -279,7 +279,7 @@
         default:
             break;
         }
-        if (!body.isEmpty()) {
+        if (!body.empty()) {
             dprintf(fd, "%.*s%s %s\n", (int)indent, "", timestamp.c_str(), body.c_str());
             body.clear();
         }