Enforce `StringSourceLocator` to be built with a `non-empty-string`

[Ocramius]

Currently, a StringSourceLocator can be constructed with an empty string, but will crash at runtime when doing so:

BetterReflection/src/SourceLocator/Type/StringSourceLocator.php

Lines 23 to 38 in f3966ba

	/**
	* @throws EmptyPhpSourceCode
	*/
	public function __construct(private string $source, Locator $astLocator)
	{
	parent::__construct($astLocator);
	if (empty($source)) {
	// Whilst an empty string is still "valid" PHP code, there is no
	// point in us even trying to parse it because we won't find what
	// we are looking for, therefore this throws an exception
	throw new EmptyPhpSourceCode(
	'Source code string was empty',
	);
	}
	}

We should really get rid of the exception, and instead enforce non-empty-string as input.