fix: correctly place unfiltered null values for line (#9326) (#9334)

## PR Description

Previously we forgot to add conditional logic for invalid values when
filtering is disabled (mark.invalid = null)

Interestingly, given area applies stack layout by default, it doesn't
suffer from the same bug that line does.

---------

Co-authored-by: GitHub Actions Bot <vega-actions-bot@users.noreply.github.com>

examples/compiled/area_invalid_null.vg.json

@@ -0,0 +1,141 @@
+{
+  "$schema": "https://vega.github.io/schema/vega/v5.json",
+  "background": "white",
+  "padding": 5,
+  "width": 200,
+  "height": 200,
+  "style": "cell",
+  "data": [
+    {
+      "name": "source_0",
+      "values": [
+        {"x": -1, "y": null},
+        {"x": 1, "y": 10},
+        {"x": 2, "y": 30},
+        {"x": 3, "y": null},
+        {"x": 4, "y": 15},
+        {"x": 5, "y": 30},
+        {"x": 6, "y": 40},
+        {"x": 7, "y": 20},
+        {"x": 10, "y": null}
+      ]
+    },
+    {
+      "name": "data_0",
+      "source": "source_0",
+      "transform": [
+        {"type": "formula", "expr": "toNumber(datum[\"x\"])", "as": "x"},
+        {
+          "type": "impute",
+          "field": "y",
+          "groupby": [],
+          "key": "x",
+          "method": "value",
+          "value": 0
+        },
+        {
+          "type": "stack",
+          "groupby": ["x"],
+          "field": "y",
+          "sort": {"field": [], "order": []},
+          "as": ["y_start", "y_end"],
+          "offset": "zero"
+        }
+      ]
+    }
+  ],
+  "marks": [
+    {
+      "name": "marks",
+      "type": "area",
+      "style": ["area"],
+      "sort": {"field": "datum[\"x\"]"},
+      "from": {"data": "data_0"},
+      "encode": {
+        "update": {
+          "orient": {"value": "vertical"},
+          "fill": {"value": "#4c78a8"},
+          "description": {
+            "signal": "\"x: \" + (format(datum[\"x\"], \"\")) + \"; y: \" + (format(datum[\"y\"], \"\"))"
+          },
+          "x": [
+            {
+              "test": "!isValid(datum[\"x\"]) || !isFinite(+datum[\"x\"])",
+              "value": 0
+            },
+            {"scale": "x", "field": "x"}
+          ],
+          "y": {"scale": "y", "field": "y_end"},
+          "y2": {"scale": "y", "field": "y_start"}
+        }
+      }
+    }
+  ],
+  "scales": [
+    {
+      "name": "x",
+      "type": "linear",
+      "domain": {"data": "data_0", "field": "x"},
+      "range": [0, {"signal": "width"}],
+      "nice": true,
+      "zero": false
+    },
+    {
+      "name": "y",
+      "type": "linear",
+      "domain": {"data": "data_0", "fields": ["y_start", "y_end"]},
+      "range": [{"signal": "height"}, 0],
+      "nice": true,
+      "zero": true
+    }
+  ],
+  "axes": [
+    {
+      "scale": "x",
+      "orient": "bottom",
+      "gridScale": "y",
+      "grid": true,
+      "tickCount": {"signal": "ceil(width/40)"},
+      "domain": false,
+      "labels": false,
+      "aria": false,
+      "maxExtent": 0,
+      "minExtent": 0,
+      "ticks": false,
+      "zindex": 0
+    },
+    {
+      "scale": "y",
+      "orient": "left",
+      "gridScale": "x",
+      "grid": true,
+      "tickCount": {"signal": "ceil(height/40)"},
+      "domain": false,
+      "labels": false,
+      "aria": false,
+      "maxExtent": 0,
+      "minExtent": 0,
+      "ticks": false,
+      "zindex": 0
+    },
+    {
+      "scale": "x",
+      "orient": "bottom",
+      "grid": false,
+      "title": "x",
+      "labelFlush": true,
+      "labelOverlap": true,
+      "tickCount": {"signal": "ceil(width/40)"},
+      "zindex": 0
+    },
+    {
+      "scale": "y",
+      "orient": "left",
+      "grid": false,
+      "title": "y",
+      "labelOverlap": true,
+      "tickCount": {"signal": "ceil(height/40)"},
+      "zindex": 0
+    }
+  ]
+}