fix: don't call onCoverage hook if there is no coverage to report

Author: sheremet-va

docs/advanced/api/reporters.md

@@ -192,6 +192,8 @@ import type { CoverageMap } from 'istanbul-lib-coverage'
 declare function onCoverage(coverage: CoverageMap): Awaitable<void>
 ```
 
+If Vitest didn't perform any coverage, this hook is not called.
+
 ## onTestModuleQueued
 
 ```ts

packages/vitest/src/node/test-run.ts

@@ -73,7 +73,9 @@ export class TestRun {
       ])
     }
     finally {
-      await this.vitest.report('onCoverage', coverage)
+      if (coverage) {
+        await this.vitest.report('onCoverage', coverage)
+      }
     }
   }