fix error with pre-release solc

[ssadler]

Pre release solc (can happen if version is not specified for example in foundry.toml) breaks the client because it emits:

    {
      component: 'general',
      errorCode: '3805',
      formattedMessage: 'Warning: This is a pre-release compiler version, please do not use it in production.\n' +
        '\n',
      message: 'This is a pre-release compiler version, please do not use it in production.',
      severity: 'warning',
      type: 'Warning'
    }

Causing Cannot read properties of undefined (reading 'file') in OutputConverter.ts, because it has no sourceLocation.