Fix #671: restrict Cucumber property pages to IResource selections

[alef75]

The Cucumber and Java Backend property pages were contributed to org.eclipse.ui.propertyPages with no objectClass/adaptable/enabledWhen filter, so they were offered for any selection. When rendered against a non-IResource element (e.g. a Remote System Explorer connection), getNode() dereferenced a null resource and threw a NullPointerException, breaking the whole Properties dialog.

• Add to both property page contributions, so they are only offered for IResource-adaptable selections.
• Make CucumberEditorProperties#getNode and CucumberJavaBackendProperties#getNode null-safe against both a null resource AND a resource with no project (the workspace root passes the IResource filter but IWorkspaceRoot#getProject() returns null, which ProjectScope otherwise rejects with an IllegalArgumentException), and guard every call site (createContents/performOk/performDefaults) that dereferences the result. getElement().getAdapter(IResource.class) can still return null for an element the adapter manager otherwise treats as adaptable, so the enablement filter alone does not fully close the NPE; the affected pages now show a not-applicable message instead of crashing.
• Align performOk()'s guard in both property pages to the control field already used by performDefaults()/createContents(), rather than recomputing getNode(), so the two stay consistent.