TriStateCheckbox doesn't show correct icon when value is false when using material themes

[tidemann]

To reproduce:

• Go to https://primefaces.org/primevue/showcase/#/tristatecheckbox.
• Select any material theme (i.e. Indigo Light).
• Toggle through null/true/false. Both true and false shows same "checked" checkbox.

[tidemann]

seems to be because of the following style in the material themes:

.p-checkbox .p-checkbox-box.p-highlight .p-checkbox-icon:before {
  content: "";
  position: absolute;
  top: 6px;
  left: 1px;
  border-right: 2px solid transparent;
  border-bottom: 2px solid transparent;
  transform: rotate(45deg);
  transform-origin: 0% 100%;
  animation: checkbox-check 125ms 50ms linear forwards;
}