Improve gracefulness when invoking wireviz.parse() without file_out

This happened to be a regression for WireViz-Web [1], which aims to do
as much in memory as possible.

[1] https://github.com/daq-tools/wireviz-web.

kvid rebased and mixed original commit with similar change by Daniel Rojas

Co-authored-by: kvid <kvid@users.noreply.github.com>

Author: amotl

src/wireviz/wireviz.py

@@ -115,8 +115,9 @@ def parse(
     # keep track of auto-generated designators to avoid duplicates
     autogenerated_designators = {}
 
+    # When title is not given, either deduce it from filename, or use default text.
     if "title" not in harness.metadata:
-        harness.metadata["title"] = Path(yaml_file).stem if yaml_file else ""
+        harness.metadata["title"] = Path(yaml_file).stem if yaml_file else "WireViz diagram and BOM"
 
     # add items
     # parse YAML input file ====================================================