Improve zero-record log messages for split and SQL statement readers

- DBTableRecordReader: log table name and split query so users can
  identify which table and range returned no records
- SQLStatementRecordReader: log the SQL statement instead of the
  resolved table name for clarity

Author: psainics

src/main/java/io/cdap/plugin/format/DBTableRecordReader.java

@@ -89,7 +89,8 @@ public boolean nextKeyValue() throws IOException {
       }
       if (!results.next()) {
         if (pos == 0) {
-          LOG.warn("Table '{}' had no records to read.", tableName.getTable());
+          LOG.warn("Split for table '{}' returned no records. Split query: '{}'",
+                   tableName.getTable(), getQuery());
         }
         return false;
       }

src/main/java/io/cdap/plugin/format/SQLStatementRecordReader.java

@@ -89,7 +89,7 @@ public boolean nextKeyValue() throws IOException {
       }
       if (!results.next()) {
         if (pos == 0) {
-          LOG.warn("Table '{}' had no records to read.", tableName);
+          LOG.warn("SQL statement '{}' returned no records.", split.getSqlStatement());
         }
         return false;
       }