refactor: Use empty() over eof() in the streams interface

End-of-file does not really make sense for streams that wrap buffers. So
replace it by the equivalent empty() checks.

src/test/dbwrapper_tests.cpp

@@ -364,7 +364,7 @@ struct StringContentsSerializer {
     {
         str.clear();
         uint8_t c{0};
-        while (!s.eof()) {
+        while (!s.empty()) {
             s >> c;
             str.push_back(c);
         }