Fallback on gray colorspace when there are no colorspace and no name in the scn/SCN arguments

It fixes #18894.

src/core/evaluator.js

@@ -2079,6 +2079,11 @@ class PartialEvaluator {
           case OPS.setFillColorN:
             cs = stateManager.state.patternFillColorSpace;
             if (!cs) {
+              if (isNumberArray(args, null)) {
+                args = ColorSpace.singletons.gray.getRgb(args, 0);
+                fn = OPS.setFillRGBColor;
+                break;
+              }
               args = [];
               fn = OPS.setFillTransparent;
               break;
@@ -2106,6 +2111,11 @@ class PartialEvaluator {
           case OPS.setStrokeColorN:
             cs = stateManager.state.patternStrokeColorSpace;
             if (!cs) {
+              if (isNumberArray(args, null)) {
+                args = ColorSpace.singletons.gray.getRgb(args, 0);
+                fn = OPS.setStrokeRGBColor;
+                break;
+              }
               args = [];
               fn = OPS.setStrokeTransparent;
               break;

test/pdfs/.gitignore

@@ -675,3 +675,4 @@
 !stamps.pdf
 !issue15096.pdf
 !issue18036.pdf
+!issue18894.pdf

test/test_manifest.json

@@ -10692,5 +10692,13 @@
     "rounds": 1,
     "type": "eq",
     "annotations": true
+  },
+  {
+    "id": "issue18894",
+    "file": "pdfs/issue18894.pdf",
+    "md5": "5cd744bf8c0800e93c22ed92d1c162f4",
+    "rounds": 1,
+    "type": "eq",
+    "talos": false
   }
 ]