Explicitly treat `null` as an empty string when interpolating.

Fixes a deprecation warning on PHP 8.1+, and saves us some unnecessary escaping calls!

Thanks for your help @scr4bble :)

Fixes #383

src/Mustache/Compiler.php

@@ -506,7 +506,7 @@ class Mustache_Compiler
 
     const VARIABLE = '
         $value = $this->resolveValue($context->%s(%s), $context);%s
-        $buffer .= %s%s;
+        $buffer .= %s($value === null ? \'\' : %s);
     ';
 
     /**

test/Mustache/Test/CompilerTest.php

@@ -56,7 +56,7 @@ class Mustache_Test_CompilerTest extends PHPUnit_Framework_TestCase
                 array(
                     "\nclass Monkey extends Mustache_Template",
                     '$value = $this->resolveValue($context->find(\'name\'), $context);',
-                    '$buffer .= $indent . call_user_func($this->mustache->getEscape(), $value);',
+                    '$buffer .= $indent . ($value === null ? \'\' : call_user_func($this->mustache->getEscape(), $value));',
                     'return $buffer;',
                 ),
             ),
@@ -76,7 +76,7 @@ class Mustache_Test_CompilerTest extends PHPUnit_Framework_TestCase
                 array(
                     "\nclass Monkey extends Mustache_Template",
                     '$value = $this->resolveValue($context->find(\'name\'), $context);',
-                    '$buffer .= $indent . htmlspecialchars($value, ' . ENT_COMPAT . ', \'ISO-8859-1\');',
+                    '$buffer .= $indent . ($value === null ? \'\' : htmlspecialchars($value, ' . ENT_COMPAT . ', \'ISO-8859-1\'));',
                     'return $buffer;',
                 ),
             ),
@@ -96,7 +96,7 @@ class Mustache_Test_CompilerTest extends PHPUnit_Framework_TestCase
                 array(
                     "\nclass Monkey extends Mustache_Template",
                     '$value = $this->resolveValue($context->find(\'name\'), $context);',
-                    '$buffer .= $indent . htmlspecialchars($value, ' . ENT_QUOTES . ', \'ISO-8859-1\');',
+                    '$buffer .= $indent . ($value === null ? \'\' : htmlspecialchars($value, ' . ENT_QUOTES . ', \'ISO-8859-1\'));',
                     'return $buffer;',
                 ),
             ),
@@ -123,7 +123,7 @@ class Mustache_Test_CompilerTest extends PHPUnit_Framework_TestCase
                     "\nclass Monkey extends Mustache_Template",
                     "\$buffer .= \$indent . 'foo\n';",
                     '$value = $this->resolveValue($context->find(\'name\'), $context);',
-                    '$buffer .= htmlspecialchars($value, ' . ENT_COMPAT . ', \'UTF-8\');',
+                    '$buffer .= ($value === null ? \'\' : htmlspecialchars($value, ' . ENT_COMPAT . ', \'UTF-8\'));',
                     '$value = $this->resolveValue($context->last(), $context);',
                     '$buffer .= \'\\\'bar\\\'\';',
                     'return $buffer;',