buffer: allow to read zero bytes

Decoder can try to read zero bytes sometimes. For example, when
it received an empty string. Let's simply handle this case in buffer
instead of assertion.

By the way, `ClientTest.cpp` was failing with Tarantool 3.2 because of
this problem: when connector tries to call a non-existing function
(`*** TEST CASE call wrong function ***`), Tarantool sends an empty
string in response.

src/Buffer/Buffer.hpp

@@ -1238,7 +1238,8 @@ template <bool LIGHT>
 void
 Buffer<N, allocator>::iterator_common<LIGHT>::read(RData data)
 {
-	assert(data.size > 0);
+	if (data.size == 0)
+		return;
 	/*
 	 * The same implementation as in ::set() method buf vice versa:
 	 * buffer and data sources are swapped.