Fixes "undefined $ld" error message with ImplicitPlane objects

This avoids error messages from comparisons used in testing for when the formula agrees with the previous one typed by the student when the correct answer is a list of implicit planes.  It also avoids error messages when a string answer is entered when a list of implicit planes is expected.

This can be tested using any problem that uses ImplicitPlane in a List by submitting two different answers.  The second one should produce the error message in an unpatched PG, and no error with this patch.  Entering a string like `NONE` should produce a warning about not being an implicit plane in an unpatched version and no error message with this patch.

Here is a sample problem

```
DOCUMENT();
loadMacros(
  "PGstandard.pl",
  "MathObjects.pl",
  "parserImplicitPlane.pl",
);

$P = ImplicitPlane("x+y=1");

Context()->texStrings;
BEGIN_TEXT
\($P\) is \{$P->ans_rule\}
END_TEXT
Context()->normalStrings;

ANS(List($P)->cmp);

ENDDOCUMENT();
```