[CARBONDATA-4213] Fix update/delete issue in index server

Why is this PR needed?
During update/delete, the segment file in the segment would come as an empty
string due to which it was not able to read the segment file.

What changes were proposed in this PR?
1. Changed the empty string to NULL
2. Added empty segment file condition while creating SegmentFileStore.

Does this PR introduce any user interface change?
No

Is any new testcase added?
No

This closes #4153

core/src/main/java/org/apache/carbondata/core/mutate/CarbonUpdateUtil.java

@@ -580,7 +580,7 @@ public static List<Segment> getListOfSegmentsToMarkDeleted(Map<String, Long> seg
     for (Map.Entry<String, Long> eachSeg : segmentBlockCount.entrySet()) {
 
       if (eachSeg.getValue() == 0) {
-        segmentsToBeDeleted.add(new Segment(eachSeg.getKey(), ""));
+        segmentsToBeDeleted.add(new Segment(eachSeg.getKey(), (String) null));
       }
 
     }

core/src/main/java/org/apache/carbondata/core/readcommitter/TableStatusReadCommittedScope.java

@@ -83,7 +83,7 @@ public LoadMetadataDetails[] getSegmentList() throws IOException {
   public Map<String, String> getCommittedIndexFile(Segment segment) throws IOException {
     Map<String, String> indexFiles;
     SegmentFileStore fileStore = null;
-    if (segment.getSegmentFileName() != null) {
+    if (segment.getSegmentFileName() != null && !segment.getSegmentFileName().isEmpty()) {
       fileStore = new SegmentFileStore(identifier.getTablePath(), segment.getSegmentFileName());
     }
     if (segment.getSegmentFileName() == null || fileStore.getSegmentFile() == null) {