The specification for this method says:
----
TypeElement getTypeElement(String name)

    Returns a type element given its fully qualified name.

    Parameters:
        name - fully qualified type name
    Returns:
        the named type element, or *null if it cannot be found*
----

However null is never returned in cases when non-existing type names are passed.
Here is an example from 6346506:
----
import java.util.*;
import javax.annotation.processing.*;
import javax.lang.model.element.*;
import javax.lang.model.type.*;
import javax.lang.model.util.*;
import static javax.lang.model.SourceVersion.*;
import static javax.lang.model.type.TypeKind.*;

@SupportedAnnotationTypes("*")
@SupportedSourceVersion(RELEASE_6)
@SupportedOptions("-verbose")
public class CaseAP extends AbstractProcessor {
    public void init(ProcessingEnvironment penv) {
        super.init(penv);
    }
    public boolean process(Set<? extends TypeElement> typeElementSet,
            RoundEnvironment renv) {
        Elements elementUtils = processingEnv.getElementUtils();
        TypeElement case2 = elementUtils.getTypeElement("non.existing.Class");
// Case2 class doesn;t exisit in classpath to construct a Element.
        if ( case2 != null ) {
            System.out.println(" element received as "+case2.toString());
        }
        return true ;
    }
}
----

$JAVA_HOME/bin/javac -classpath $CLASSPATH -processor CaseAP Case1.java
 element received as non.existing.Class
 element received as non.existing.Class

Java(TM) SE Runtime Environment (build 1.6.0-beta2-b77)