fix: prefer user-defined tickMinStep to defaultTickMinStep

[datananda]

PR Description

Fixes #9034 by only using defaultTickMinStep if the user hasn't explicitly provided a tickMinStep.

example 1

{
  "$schema": "https://vega.github.io/schema/vega-lite/v4.json",
  "description": "A simple bar chart with embedded data.",
  "data": {
    "values": [
      {"a": "A", "b": 2}, {"a": "B", "b": 1}, {"a": "C", "b": 1},
      {"a": "D", "b": 1}, {"a": "E", "b": 2}, {"a": "F", "b": 2},
      {"a": "G", "b": 3}, {"a": "H", "b": 3}, {"a": "I", "b": 1}
    ]
  },
  "mark": "bar",
  "encoding": {
    "x": {"field": "a", "type": "nominal", "axis": {"labelAngle": 0}},
    "y": {"field": "b", "type": "quantitative", "axis": {"tickMinStep": 1}}
  }
}

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example 2

{
  "config": {"view": {"continuousWidth": 300, "continuousHeight": 300}},
  "data": {"name": "data-4a0cb25b0716723cd309daa74d88bb73"},
  "mark": {"type": "point"},
  "encoding": {
    "x": {"axis": {"tickMinStep": 1}, "field": "x", "type": "quantitative"},
    "y": {"field": "y", "type": "quantitative"}
  },
  "$schema": "https://vega.github.io/schema/vega-lite/v5.8.0.json",
  "datasets": {
    "data-4a0cb25b0716723cd309daa74d88bb73": [
      {"x": 0, "y": 0},
      {"x": 1, "y": 1},
      {"x": 2, "y": 2},
      {"x": 3, "y": 3},
      {"x": 4, "y": 4}
    ]
  }
}

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example 3

{
  "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
  "data": {"url": "data/movies.json"},
  "mark": "bar",
  "encoding": {
    "x": {
      "bin": true,
      "field": "IMDB Rating",
      "axis": {
        "grid": true,
        "tickMinStep": 2
      }
    },
    "y": {"aggregate": "count"}
  }
}

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