fixed warnings in Font.php

Warnings were:

1) /var/www/html/src/Smalot/PdfParser/Font.php:292
range(): Argument #1 ($start) must not be empty, casted to 0

Triggered by:

* PHPUnitTests\Integration\FontTest::testCalculateTextWidthNoWidthsKey
  /var/www/html/tests/PHPUnit/Integration/FontTest.php:473

2) /var/www/html/src/Smalot/PdfParser/Font.php:292
range(): Argument #2 ($end) must not be empty, casted to 0

Triggered by:

* PHPUnitTests\Integration\FontTest::testCalculateTextWidthNoWidthsKey
  /var/www/html/tests/PHPUnit/Integration/FontTest.php:473

src/Smalot/PdfParser/Font.php

@@ -288,7 +288,8 @@ public function calculateTextWidth(string $text, ?array &$missing = null): ?floa
         $widths = $details['Widths'] ?? [];
 
         // Widths array is zero indexed but table is not. We must map them based on FirstChar and LastChar
-        $width_map = array_flip(range($details['FirstChar'], $details['LastChar']));
+        // note: in some cases the values for FirstChar or LastChar are null here
+        $width_map = array_flip(range((int) $details['FirstChar'], (int) $details['LastChar']));
 
         $width = null;
         $missing = [];