Fix NP in containsCell in TableViews

This pull request addresses a nullpointer if you use containsCell("...") and there is no match in the table.

TableView.fxml:

```
<TableView fx:id="entryTableView" prefHeight="199.0" prefWidth="290.0">
              <columns>
                <TableColumn prefWidth="140.0" text="Entry" fx:id="entryColumn" />
                <TableColumn maxWidth="5000.0" minWidth="10.0" prefWidth="55.0" text="Default" fx:id="defaultColumn" />
                <TableColumn prefWidth="75.0" text="Navigation">
                  <columns>
                    <TableColumn prefWidth="50.0" text="UP" fx:id="upColumn" />
                    <TableColumn prefWidth="50.0" text="DOWN" fx:id="downColumn" />
                    <TableColumn prefWidth="50.0" style="" text="DEL." fx:id="removeColumn" />
                  </columns>
                </TableColumn>
              </columns>
            </TableView>
```

Test code:

```
  verifyThat(GuiTest.find("#entryTableView"), containsCell("item0"))
```
Stacktrace:

```
java.lang.NullPointerException
	at org.loadui.testfx.controls.TableViews.containsCell(TableViews.java:83)
	at org.loadui.testfx.controls.TableViews$TableContainsMatcher.matches(TableViews.java:184)
	at org.hamcrest.MatcherAssert.assertThat(MatcherAssert.java:12)
	at org.loadui.testfx.Assertions.verifyThat(Assertions.java:32)
	at org.loadui.testfx.Assertions.verifyThat(Assertions.java:25)
	at ....
```