Treat empty string as an empty stage

Instead of trying to validate an empty string as a potential valid
stage, treat it as if no stage was provided.

This fixes #60.

reckon-core/src/main/java/org/ajoberstar/reckon/core/strategy/StagePreReleaseStrategy.java

@@ -48,7 +48,10 @@ public StagePreReleaseStrategy(
 
   @Override
   public Version reckonTargetVersion(VcsInventory inventory, Version targetNormal) {
-    String stage = stageCalc.apply(inventory, targetNormal).orElse(null);
+    String stage = stageCalc.apply(inventory, targetNormal)
+      .map(String::trim)
+      .filter(s -> !s.isEmpty())
+      .orElse(null);
 
     if (stage != null && !stages.contains(stage)) {
       String message = String.format("Stage \"%s\" is not one of: %s", stage, stages);

reckon-core/src/test/groovy/org/ajoberstar/reckon/core/strategy/StagePreReleaseStrategyTest.groovy

@@ -47,6 +47,11 @@ class StagePreReleaseStrategyTest extends Specification {
     strategy(null).reckonTargetVersion(inventory, Version.valueOf('2.0.0')) == Version.valueOf('2.0.0-initial.0.5+abcdef')
   }
 
+  def 'if target does not contain stage and stage is an empty string, use the default stage and add num commits and commit id'() {
+    expect:
+    strategy('').reckonTargetVersion(inventory, Version.valueOf('2.0.0')) == Version.valueOf('2.0.0-initial.0.5+abcdef')
+  }
+
   def 'if target does not contain stage and stage is present, add num commits and commit id'() {
     expect:
     strategy(null).reckonTargetVersion(inventory, Version.valueOf('1.2.3')) == Version.valueOf('1.2.3-milestone.2.5+abcdef')