Fix generated properties file NPE.

- Adds a test case to verify that the written properties file can be read again.

Author: ryantse

core/src/main/java/pl/project13/core/PropertiesFileGenerator.java

@@ -75,8 +75,8 @@ public void maybeGeneratePropertiesFile(@Nonnull Properties localProperties, Fil
 
           final String buildTimeProperty = prefixDot + GitCommitPropertyConstant.BUILD_TIME;
 
-          propertiesCopy.setProperty(buildTimeProperty, null);
-          persistedProperties.setProperty(buildTimeProperty, null);
+          propertiesCopy.setProperty(buildTimeProperty, "");
+          persistedProperties.setProperty(buildTimeProperty, "");
 
           shouldGenerate = !propertiesCopy.equals(persistedProperties);
         } catch (CannotReadFileException ex) {

maven/src/test/java/pl/project13/core/PropertiesFileGeneratorTest.java

@@ -65,4 +65,24 @@ public void generatedPropertiesFileDoesNotContainDateComment() throws GitCommitI
         + "commit.id.full=b5993378ffadd1f84dc8da220b9204d157ec0f29\n";
     assertEquals(expectedContent, actualContent);
   }
+
+  @Test
+  public void rereadGeneratedPropertiesFile() throws GitCommitIdExecutionException, IOException {
+    Properties properties = new Properties();
+    properties.put(GitCommitPropertyConstant.COMMIT_ID_FULL, "b5993378ffadd1f84dc8da220b9204d157ec0f29");
+    properties.put(GitCommitPropertyConstant.BRANCH, "develop");
+  
+    Path propertiesPath = temporaryFolder.getRoot().toPath().resolve("git.properties");
+    propertiesFileGenerator.maybeGeneratePropertiesFile(properties, temporaryFolder.getRoot(), propertiesPath.getFileName().toString(), UTF_8);
+
+    // Re-read the generated properties file.
+    propertiesFileGenerator.maybeGeneratePropertiesFile(properties, temporaryFolder.getRoot(), propertiesPath.getFileName().toString(), UTF_8);
+
+    byte[] bytes = Files.readAllBytes(propertiesPath);
+    String actualContent = new String(bytes, UTF_8);
+    String expectedContent = "#Generated by Git-Commit-Id-Plugin\n"
+        + "branch=develop\n"
+        + "commit.id.full=b5993378ffadd1f84dc8da220b9204d157ec0f29\n";
+    assertEquals(expectedContent, actualContent);
+  }
 }