Do not touch the outputTimeStamp at all if SOURCE_DATE_EPOCH is not set

Author: fridrich

src/main/java/org/apache/maven/plugins/javadoc/AbstractJavadocMojo.java

@@ -2668,10 +2668,13 @@ private void populateCompileArtifactMap(Map<String, Artifact> compileArtifactMap
     private String getBottomText() {
         final String inceptionYear = project.getInceptionYear();
 
-        if (outputTimestamp == null
-                || outputTimestamp.length() < 1
-                || ((outputTimestamp.length() == 1) && !Character.isDigit(outputTimestamp.charAt(0)))) {
-            outputTimestamp = System.getenv("SOURCE_DATE_EPOCH");
+        final String sde = System.getenv("SOURCE_DATE_EPOCH");
+        if (sde != null) {
+            if (outputTimestamp == null
+                    || outputTimestamp.length() < 1
+                    || ((outputTimestamp.length() == 1) && !Character.isDigit(outputTimestamp.charAt(0)))) {
+                outputTimestamp = sde;
+            }
         }
 
         // get Reproducible Builds outputTimestamp date value or the current local date.