DocBlock/Tags/Source: remove redundant code
DocBlock/Tags/Source: remove redundant code

PHPStan flags the code within the `Source::__toString()` method:
```
 ------ -------------------------------------------------------------------------------------------
  Line   DocBlock\Tags\Source.php
 ------ -------------------------------------------------------------------------------------------
  111    Result of || is always true.
  114    Result of || is always true.
  114    Result of || is always true.
 ------ -------------------------------------------------------------------------------------------
```

I have investigated this and can confirm that these flags are correct.

1. `$this->startingLine` is cast to an integer in the `__construct()` method (line 45) and subsequently cast to a string in `__toString()` (line 105).
    This means that it can only ever be a non-empty ("truthy") string or the string '0', so the `$startingLine || $startingLine === '0'` condition used in two places is redundant.
2. `$this->lineCount` is either an integer or `null` after the `__construct()` method (line 46).
    In the `__toString()` method, if the `lineCount` is an integer, it is effectively cast to a string by the concatenation with an empty string on line 107, while if the `lineCount` was `null`, it is turned into an empty string.
    By changing the concatenation from concatenating with an empty string to concatenating with a one-space string, we can remove the ternary in the `return` statement checking for `$lineCount` being empty.

The existing unit tests already cover this code and still pass after this change.

phpDocumentor/ReflectionDocBlock