DocBlock/Tags/Source: remove redundant code

[jrfnl]

PHPStan flags the code within the Source::__toString() method:

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  Line   DocBlock\Tags\Source.php
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  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.