Handle null hparam values in table view. (#6516)

Hparams from data provider can return null for some hparam values. The
Hparams dashboard code does not yet expect this and would call
toString() on null values, resulting in error and stopping the table
from rendering properly.

The fix is simple, render empty string '' for null values just as we
already do for undefined values.

Author: bmd3k

tensorboard/plugins/hparams/tf_hparams_utils/tf-hparams-utils.ts

@@ -302,7 +302,7 @@ export function prettyPrint(value) {
     // TODO(erez):Make the precision user-configurable.
     return value.toPrecision(5);
   }
-  if (value === undefined) {
+  if (value === null || value === undefined) {
     return '';
   }
   return value.toString();