Proposed Fix of Return value of PHPUnit\Framework\TestCase::getExpectedExceptionMessage() must be of the type string, null returned

[slaughter550]

Q	A
PHPUnit version	7.5.1
PHP version	7.2.11
Installation Method	Composer

In https://github.com/sebastianbergmann/phpunit/blob/7.5/src/Framework/TestCase.php, there are the following variable declarations

    /**
     * @var null|string
     */
    private $expectedException;

    /**
     * @var string
     */
    private $expectedExceptionMessage;

    /**
     * @var string
     */
    private $expectedExceptionMessageRegExp;

and the following getters

    public function getExpectedException(): ?string
    {
        return $this->expectedException;
    }

    public function getExpectedExceptionMessage(): string
    {
        return $this->expectedExceptionMessage;
    }

    public function getExpectedExceptionMessageRegExp(): string
    {
        return $this->expectedExceptionMessageRegExp;
    }

Due to the fact that both expectedExceptionMessage and expectedExceptionMessageRegExp are declared but not initialized on instantiation, there is a chance that when called, they violate their requirement to return a string.

Proposed solution. Make them optionally return a string just like expectedException allows for.

@sebastianbergmann thoughts? I'm happy to produce the PR if you are good with this direction.

[slaughter550]

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