compute proposals for blank strings

Before we would only provide proposals for empty strings.
But blank and empty is not that different.

data-store/src/main/java/org/lucares/pdb/datastore/internal/Proposer.java

@@ -6,6 +6,7 @@ import java.util.Map.Entry;
 import java.util.SortedSet;
 import java.util.TreeSet;
 
+import org.apache.commons.lang3.StringUtils;
 import org.lucares.pdb.datastore.Proposal;
 import org.lucares.pdb.datastore.lang.ProposerParser;
 import org.lucares.utils.CollectionUtils;
@@ -21,7 +22,7 @@ public class Proposer {
 	public List<Proposal> propose(final String query, final int caretIndex) {
 		final SortedSet<Proposal> result;
 
-		if (query.isEmpty()) {
+		if (StringUtils.isBlank(query)) {
 			result = proposeForAllKeys();
 		} else {
 			result = ProposerParser.parse(query, dataStore, caretIndex);