Assume x values for missing signal data in FST

[RCoeurjoly]

What are the reasons/motivation for this change?
Handle cases where signal data is missing in FST files during simulation. Previously, when a signal ID was not found in past_data, it would cause an error. This patch ensures that missing signal data is assumed to be undefined ('x'), which aligns with the behaviour of gtkwave. Below is a screenshot of how gtkwave interprets tests/sim/simple_assign.vcd.

Explain how this is achieved.
Instead of logging an error when the signal ID is not found, the valueOf now returns a string filled with 'x' characters, where the length of the string corresponds to the signal’s width.
If applicable, please suggest to reviewers how they can test the change.

Note

I considered adding a warning like:
log_warning("Signal id %d not found, assuming x\n", (int)signal);
but decided against it to avoid extra verbosity. Open to adding it if needed.

[mmicko]

Think this we can safely add after release