The method ```io.modelcontextprotocol.client.McpAsyncClient#listTools()``` never returns if the ```result.nextCursor()``` is an empty string instead of a null

[jthann]

I've found in some MCP server implementations the nextCursor in response is empty string instead of null, leading to dead loop in the method of following reference.

java-sdk/mcp-core/src/main/java/io/modelcontextprotocol/client/McpAsyncClient.java

Line 557 in 7f16cd0

.expand(result -> (result.nextCursor() != null) ? this.listTools(result.nextCursor()) : Mono.empty())