Gracefully handle empty nested testsuite elements for JUnit.

This fixes an issue with mocha-junit-reporter returning empty root-level testsuite elements.

__tests__/fixtures/empty/jest-junit-empty-testsuite.xml

@@ -0,0 +1,5 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<testsuites name="jest tests" tests="0" failures="0" errors="0" time="11.299">
+	<testsuite name="__tests__\main.test.js" errors="0" failures="0" skipped="0" timestamp="2020-10-27T21:39:41" time="0.486" tests="0">
+  	</testsuite>
+</testsuites>

__tests__/jest-junit.test.ts

@@ -7,7 +7,7 @@ import {getReport} from '../src/report/get-report'
 import {normalizeFilePath} from '../src/utils/path-utils'
 
 describe('jest-junit tests', () => {
-  it('produces empty test run result when there are no test cases', async () => {
+  it('produces empty test run result when there are no test cases in the testsuites element', async () => {
     const fixturePath = path.join(__dirname, 'fixtures', 'empty', 'jest-junit.xml')
     const filePath = normalizeFilePath(path.relative(__dirname, fixturePath))
     const fileContent = fs.readFileSync(fixturePath, {encoding: 'utf8'})
@@ -23,6 +23,22 @@ describe('jest-junit tests', () => {
     expect(result.result).toBe('success')
   })
 
+  it('produces empty test run result when there are no test cases in a nested testsuite element', async () => {
+    const fixturePath = path.join(__dirname, 'fixtures', 'empty', 'jest-junit-empty-testsuite.xml')
+    const filePath = normalizeFilePath(path.relative(__dirname, fixturePath))
+    const fileContent = fs.readFileSync(fixturePath, {encoding: 'utf8'})
+
+    const opts: ParseOptions = {
+      parseErrors: true,
+      trackedFiles: []
+    }
+
+    const parser = new JestJunitParser(opts)
+    const result = await parser.parse(filePath, fileContent)
+    expect(result.tests).toBe(0)
+    expect(result.result).toBe('success')
+  })
+
   it('report from ./reports/jest test results matches snapshot', async () => {
     const fixturePath = path.join(__dirname, 'fixtures', 'jest-junit.xml')
     const outputPath = path.join(__dirname, '__outputs__', 'jest-junit.md')

src/parsers/jest-junit/jest-junit-parser.ts

@@ -48,6 +48,10 @@ export class JestJunitParser implements TestParser {
   }
 
   private getGroups(suite: TestSuite): TestGroupResult[] {
+    if (!suite.testcase) {
+      return []
+    }
+
     const groups: {describe: string; tests: TestCase[]}[] = []
     for (const tc of suite.testcase) {
       let grp = groups.find(g => g.describe === tc.$.classname)

src/parsers/jest-junit/jest-junit-types.ts

@@ -19,7 +19,7 @@ export interface TestSuite {
     time: string
     timestamp?: Date
   }
-  testcase: TestCase[]
+  testcase?: TestCase[]
 }
 
 export interface TestCase {