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/net.cpp

@@ -202,7 +202,7 @@ static std::vector<CAddress> ConvertSeeds(const std::vector<uint8_t> &vSeedsIn)
     std::vector<CAddress> vSeedsOut;
     FastRandomContext rng;
     ParamsStream s{DataStream{vSeedsIn}, CAddress::V2_NETWORK};
-    while (!s.eof()) {
+    while (!s.empty()) {
         CService endpoint;
         s >> endpoint;
         CAddress addr{endpoint, SeedsServiceFlags()};

src/serialize.h

@@ -1127,7 +1127,7 @@ public:
     void write(std::span<const std::byte> src) { GetStream().write(src); }
     void read(std::span<std::byte> dst) { GetStream().read(dst); }
     void ignore(size_t num) { GetStream().ignore(num); }
-    bool eof() const { return GetStream().eof(); }
+    bool empty() const { return GetStream().empty(); }
     size_t size() const { return GetStream().size(); }
 
     //! Get reference to stream parameters.

src/streams.h

@@ -195,7 +195,6 @@ public:
     //
     // Stream subset
     //
-    bool eof() const             { return size() == 0; }
     int in_avail() const         { return size(); }
 
     void read(std::span<value_type> dst)

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);
         }

src/wallet/walletdb.cpp

@@ -377,7 +377,7 @@ bool LoadCryptedKey(CWallet* pwallet, DataStream& ssKey, DataStream& ssValue, st
 
         // Get the checksum and check it
         bool checksum_valid = false;
-        if (!ssValue.eof()) {
+        if (!ssValue.empty()) {
             uint256 checksum;
             ssValue >> checksum;
             if (!(checksum_valid = Hash(vchPrivKey) == checksum)) {