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>

nodes/testpattern/main.cpp

@@ -27,7 +27,7 @@ class TestPatternNode : public dmf::NodeBase {
         mxlStatus vst = mxlCreateFlowWriter(
             instance(),
             dmf::make_video_flow_def(flow_id, node_id(), width, height, fps_num, fps_den).c_str(),
-            nullptr, &video_writer, &video_cfg, &created);
+            "", &video_writer, &video_cfg, &created);
         if (vst != MXL_STATUS_OK) { log("video mxlCreateFlowWriter failed (%s)", dmf::mxl_status_str(vst)); return; }
 
         const uint32_t video_stride = video_cfg.discrete.sliceSizes[0];
@@ -53,7 +53,7 @@ class TestPatternNode : public dmf::NodeBase {
                 instance(),
                 dmf::make_audio_flow_def(audio_flow_id, node_id(), sample_rate, channels, 32,
                                          fps_num, fps_den).c_str(),
-                nullptr, &audio_writer, &audio_cfg, &created);
+                "", &audio_writer, &audio_cfg, &created);
             if (ast != MXL_STATUS_OK) {
                 log("audio mxlCreateFlowWriter failed (%s) — continuing without audio", dmf::mxl_status_str(ast));
                 has_audio = false;