Improve null/empty String checks.

Improve null/empty String checks.

* pom.xml: Add a dependency on commons-lang, to use
org.apache.commons.lang.StringUtils.
* src/main/java/org/glom/web/server/ConfiguredDocument.java
* src/main/java/org/glom/web/server/OnlineGlomServiceImpl.java
* src/main/java/org/glom/web/server/database/RelatedListDBAccess.java
* src/main/java/org/glom/web/server/database/RelatedListNavigation.java:
Use StringUtils.isEmpty().

* src/main/java/org/glom/web/client/StringUtils.java: Add a tiny
StringUtils class with a static isEmpty() function because we
cannot use org.apache.commons.lang.StringUtils in client-side
GWT code because it (apparently) cannot be compiled to javascript.
* src/main/java/org/glom/web/client/activity/DetailsActivity.java
* src/main/java/org/glom/web/client/activity/ListActivity.java
* src/main/java/org/glom/web/client/activity/TableSelectionActivity.java
* src/main/java/org/glom/web/client/place/DetailsPlace.java
* src/main/java/org/glom/web/client/place/HasSelectableTablePlace.java
* src/main/java/org/glom/web/client/place/ListPlace.java
* src/main/java/org/glom/web/client/ui/cell/NumericCell.java
* src/main/java/org/glom/web/client/ui/cell/TextCell.java
* src/main/java/org/glom/web/client/ui/details/DetailsCell.java
* src/main/java/org/glom/web/client/ui/details/Group.java
* src/main/java/org/glom/web/client/ui/details/Notebook.java: Use
our StringUtils.isEmpty() function.