fix: MXL_ERR_FLOW_INVALID reconnect, nullptr→"" options, null guards

Readers (ndiout, fakesink) now handle MXL_ERR_FLOW_INVALID by releasing
and recreating the flow reader, then realigning the index to current time.
This lets consumer nodes survive a producer restart without exiting.

All mxlCreateInstance/FlowWriter/FlowReader options args changed from
nullptr to "" to match MXL reference implementation style.

mxlReleaseFlowReader calls guarded with null checks so cleanup is safe
when a mid-run reconnect attempt fails.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

shared/NodeBase.hpp

@@ -80,7 +80,7 @@ public:
 
         log("domain=%s", domain_.c_str());
 
-        inst_ = mxlCreateInstance(domain_.c_str(), nullptr);
+        inst_ = mxlCreateInstance(domain_.c_str(), "");
         if (!inst_) {
             log("mxlCreateInstance failed at %s", domain_.c_str());
             return 1;