Explicit model import bugfix (when no .sta file is provided).

With accompanying regression test. Follows additional fix in last commit.

prism-tests/functionality/import/dice2.lab

@@ -0,0 +1,8 @@
+0="init" 1="deadlock" 2="end" 3="six"
+0: 0
+7: 2
+8: 2
+9: 2
+10: 2
+11: 2
+12: 2 3

prism-tests/functionality/import/dice2.pm.importexport.auto

@@ -0,0 +1,3 @@
+-dtmc -importmodel dice2.tra,lab,srew -exportmodel dice2.sta,tra,lab,srew
+-dtmc -importmodel dice2.tra,lab,srew -exportmodel dice2.sta,tra,lab,srew -ex
+-importmodel dice2.tra,lab,srew -exportmodel dice2.sta,tra,lab,srew -ex

prism-tests/functionality/import/dice2.srew

@@ -0,0 +1,8 @@
+13 7
+0 1
+1 1
+2 1
+3 1
+4 1
+5 1
+6 1

prism-tests/functionality/import/dice2.sta

@@ -0,0 +1,14 @@
+(x)
+0:(0)
+1:(1)
+2:(2)
+3:(3)
+4:(4)
+5:(5)
+6:(6)
+7:(7)
+8:(8)
+9:(9)
+10:(10)
+11:(11)
+12:(12)

prism-tests/functionality/import/dice2.tra

@@ -0,0 +1,21 @@
+13 20
+0 1 0.5
+0 2 0.5
+1 3 0.5
+1 4 0.5
+2 5 0.5
+2 6 0.5
+3 1 0.5
+3 7 0.5
+4 8 0.5
+4 9 0.5
+5 10 0.5
+5 11 0.5
+6 2 0.5
+6 12 0.5
+7 7 1
+8 8 1
+9 9 1
+10 10 1
+11 11 1
+12 12 1

prism/src/prism/ExplicitFiles2ModelInfo.java

@@ -102,6 +102,8 @@ public class ExplicitFiles2ModelInfo extends PrismComponent
 		// This way, expressions can refer to the labels later on.
 		if (labelsFile != null) {
 			extractLabelNamesFromLabelsFile(labelsFile);
+		} else {
+			labelNames = new ArrayList<>();
 		}
 		
 		// Set model type: if no preference stated, try to autodetect