Fix for NPE when CompileException has a null expression

Thread [main] (Suspended (breakpoint at line 12 in ErrorUtil))	
	owns: DefaultAgenda  (id=64)	
	ErrorUtil.rewriteIfNeeded(CompileException, char[], int) line: 12	
	ExpressionCompiler._compile() line: 302	
	ExpressionCompiler.compile(ParserContext) line: 62	
	MVEL.compileExpression(String, ParserContext) line: 810	

Offending expression:
$row.checkValueEquals(getCode__c(), "Code", null) && ( ! (
                $row.checkValueInRange(getQty__c(), "LineItemQty", null)
                // && condition 2
                // && condition 3
            ) ) && SVMXC__Service_Order__c == $serviceOrder.Id

Error message:
[Error: incompatible types in statement: class java.lang.String (compared from: class java.lang.Class)]
[Near : {... Unknown ....}]
             ^
[Line: 1, Column: 0]