No output with multiple reporters

Using the NodeJS API, let's say our `config` is like this:

```
let KarmaConfig = {
plugins: [
'karma-chai',
'karma-mocha',
'karma-sourcemap-loader',
'karma-webpack',
'karma-mocha-reporter',
'karma-chrome-launcher'
],
singleRun: true,
autoWatch: false,
frameworks: ['mocha', 'chai', 'webpack'],
reporters: ['mocha'],
// . . .
};
```

If I add the `coverage` -stuff, everything works fine:

```
KarmaConfig.plugins.unshift('karma-coverage');
KarmaConfig.reporters.push('coverage');
KarmaConfig.coverageReporter = { // https://github.com/karma-runner/karma-coverage
dir: 'test/coverage',
reporters: [
{ type: 'text' },
{ type: 'text-summary' },
{ type: 'lcovonly', subdir: '.', file: 'istanbul-coverage.txt' }
]
};

```

**However, if I add stuff like `sonarqube` the code coverage reporting stops working (results in an empty coverage report; `mocha` and `sonarqube` reporters work fine):**

```
KarmaConfig.plugins.unshift('karma-sonarqube-reporter');
KarmaConfig.reporters.push('sonarqube');
KarmaConfig.sonarqubeReporter = { // https://github.com/fadc80/karma-sonarqube-reporter
basePath: 'test',
outputFolder: 'test/coverage',
filePattern: '**/*.js',
encoding: 'utf-8',
reportName: 'sonarqube-execution.xml'
}
```

All this is with:
```
"karma": "6.3.19"
"karma-chai": "0.1.0"
"karma-chrome-launcher": "3.1.1"
"karma-coverage": "2.2.0"
"karma-mocha": "2.0.1"
"karma-mocha-reporter": "2.2.5"
"karma-sonarqube-reporter": "1.4.0"
"karma-sourcemap-loader": "0.3.8"
"karma-webpack": "5.0.0"
```
Anyone else experiencing this kind of issue?

Start by reproducing the reported NodeJS API configuration with karma-coverage and karma-sonarqube-reporter, using the package versions listed in the issue. Compare the reporter combination with the working mocha-and-coverage setup. Done means the coverage report is no longer empty when sonarqube is enabled, with the existing coverage outputs still generated.

Written by the indexing model from the issue text.