Skip dummy subtest when EnvMatrix expands to empty (#4893)

## Changes

When all EnvMatrix values are empty lists (e.g.,
`DATABRICKS_BUNDLE_ENGINE = []`), run the test directly without creating
a `#00` dummy subtest.

Before:
```
--- PASS: TestAccept/bundle/foo (0.00s)
    --- PASS: TestAccept/bundle/foo/#00 (1.23s)
```

After:
```
--- PASS: TestAccept/bundle/foo (1.23s)
```

This allows tests to opt out of the default engine matrix by setting
`DATABRICKS_BUNDLE_ENGINE = []` without creating a confusingly-named
subtest.

## Tests

Added selftests:
- `selftest/envmatrix_empty` — verifies no subtest is created when all
EnvMatrix values are empty
- `selftest/envmatrix_mixed` — verifies empty vars are dropped while
non-empty vars still create subtests

Author: denik

acceptance/acceptance_test.go

@@ -416,14 +416,20 @@ func testAccept(t *testing.T, inprocessMode bool, singleTest string) int {
 				expanded = internal.SubsetExpanded(expanded, dir, scriptUsesEngine)
 			}
 
-			for ind, envset := range expanded {
-				envname := strings.Join(envset, "/")
-				t.Run(envname, func(t *testing.T) {
-					if runParallel {
-						t.Parallel()
-					}
-					runTest(t, dir, ind, coverDir, repls.Clone(), config, envset, envFilters)
-				})
+			// If the matrix expands to a single empty envset, run the test directly
+			// without creating a subtest (avoids the "#00" dummy subtest name).
+			if len(expanded) == 1 && len(expanded[0]) == 0 {
+				runTest(t, dir, 0, coverDir, repls.Clone(), config, nil, envFilters)
+			} else {
+				for ind, envset := range expanded {
+					envname := strings.Join(envset, "/")
+					t.Run(envname, func(t *testing.T) {
+						if runParallel {
+							t.Parallel()
+						}
+						runTest(t, dir, ind, coverDir, repls.Clone(), config, envset, envFilters)
+					})
+				}
 			}
 		})
 	}

acceptance/selftest/envmatrix_empty/out.test.toml

@@ -0,0 +1 @@
+hello

acceptance/selftest/envmatrix_empty/output.txt

@@ -0,0 +1 @@
+echo hello

acceptance/selftest/envmatrix_empty/script

@@ -0,0 +1,4 @@
+Local = true
+
+[EnvMatrix]
+DATABRICKS_BUNDLE_ENGINE = []

acceptance/selftest/envmatrix_empty/test.toml

@@ -0,0 +1 @@
+FIRST=[FIRST]

acceptance/selftest/envmatrix_mixed/out.test.toml

@@ -0,0 +1 @@
+echo "FIRST=$FIRST"

acceptance/selftest/envmatrix_mixed/output.txt

@@ -0,0 +1,6 @@
+Local = true
+
+[EnvMatrix]
+DATABRICKS_BUNDLE_ENGINE = []
+FIRST = ["alpha", "beta"]
+SECOND = []