Fix Solution.get_data_dict() Step length mismatch with starting_solution (#4990)

[kamal20122012]

Summary

• When a Simulation is resumed via starting_solution and the very first experiment step's event triggers at the cycle's initial conditions, the simulation loop appends a pybamm.EmptySolution placeholder (with a single time equal to start_time) to that cycle's step list. Solution.__add__ deduplicates that boundary timestamp against the carried-over last_state, so the placeholder time is not present in cycle.t.
• Solution.get_data_dict previously concatenated np.ones_like(step.t) for every entry in cycle.steps including the EmptySolution, which inflated data["Step"] by exactly one element per such cycle relative to data["Cycle"] and data["Time [s]"].
• Skip EmptySolution entries when building "Step" so its length stays consistent with cycle.t.

Fixes #4990

Test plan

• Reproduction script from the issue: Step, Cycle, and Time [s] are all 1763 (previously 1764 / 1763 / 1763).
• New regression test tests/unit/test_solvers/test_solution.py::TestSolution::test_get_data_cycles_steps_skips_empty_step — verified to fail on pre-fix code (Step=7 vs Cycle=6) and pass with the fix.
• Existing test_get_data_cycles_steps still passes.
• pytest tests/unit/test_solvers/test_solution.py — 55 passed (1 pre-existing skipped: test_plot requires optional matplotlib).
• pre-commit run --files <changed files> — all hooks pass.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.15%. Comparing base (19c2d57) to head (f663e97).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5479   +/-   ##
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  Coverage   98.15%   98.15%           
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  Files         338      338           
  Lines       31115    31124    +9     
=======================================
+ Hits        30542    30551    +9     
  Misses        573      573           

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