bug #1276 FileByteStream compatibility with PHP 7.4 (Daniel Schischkin)

This PR was squashed before being merged into the 6.3-dev branch.

Discussion
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FileByteStream compatibility with PHP 7.4

| Q                  | A
| ------------- | ---
| Bug fix?         | yes
| New feature?  | no
| Doc update?   | no
| BC breaks?    | no
| Deprecations? | yes
| Fixed tickets | #
| License       | MIT

Since the deprecation of returning an empty string in the `fread` function when an error occurs, checking for `false` is required for compatibility with PHP7.4.

See: https://www.php.net/manual/en/migration74.incompatible.php#migration74.incompatible.core.fread-fwrite

Commits
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64a7773 FileByteStream compatibility with PHP 7.4

lib/classes/Swift/ByteStream/FileByteStream.php

@@ -81,7 +81,7 @@ public function read($length)
 
             // If we read one byte after reaching the end of the file
             // feof() will return false and an empty string is returned
-            if ('' === $bytes && feof($fp)) {
+            if ((false === $bytes || '' === $bytes) && feof($fp)) {
                 $this->resetReadHandle();
 
                 return false;