[ZEPPELIN-1103] Fix "stackedAreaChart", "lineWithFocusChart" graph when dataset contains "NULL" value.

[voyageth]

What is this PR for?

Fix "stackedAreaChart", "lineWithFocusChart" graph when dataset contains "NULL" value.

• wired graph with "NULL" value
  
  
• fixed graph with "NULL" value
  
  

What type of PR is it?

Bug Fix

Todos

• - Check side effect.
• - Need fix when maxRowValue == rowIdx

What is the Jira issue?

• https://issues.apache.org/jira/browse/ZEPPELIN-1103

How should this be tested?

• This problem can regenerate under dataset like this.
  
• rowIndexValue variable is like this on 1777 line.
  rowIndexValue : Object {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", 13: "13", 14: "14", 15: "15", 16: "16", 17: "17", 18: "18", 19: "19", 20: "20", 21: "21", 22: "22", 23: "23", 24: "24", 25: "25", 26: "26", 27: "27", 28: "28", 29: "29", 30: "30", 31: "31", 32: "32", 33: "33", 34: "34", 35: "35", 36: "36", 37: "37", 38: "38", 39: "39", 40: "40", 41: "41", 42: "42", 43: "43", 44: "52"}
  rowNameIndex : Object {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, 13: 13, 14: 14, 15: 15, 16: 16, 17: 17, 18: 18, 19: 19, 20: 20, 21: 21, 22: 22, 23: 23, 24: 24, 25: 25, 26: 26, 27: 27, 28: 28, 29: 29, 30: 30, 31: 31, 32: 32, 33: 33, 34: 34, 35: 35, 36: 36, 37: 37, 38: 38, 39: 39, 40: 40, 41: 41, 42: 42, 43: 43, 52: 44}

[corneadoug]

@voyageth Thank you for the contribution,
Could you create a jira issue here: https://issues.apache.org/jira/browse/ZEPPELIN
And also provide better details on the bug, it could be:

• A screenshot of GIF of the bug
• Explanations of what is happening and what is expected
  As well as some code to input in a paragraph and instructions to reproduce the error

[Leemoonsoo]

Thanks @voyageth for the contribution.

I tried reproduce the problem by doing

%sh echo -e "%table key\tvalue\n\t1000\n1\t100\n2\t200"

And i've got weird graph.

However, the weird graph still remains after applying this proposed fix.
@voyageth, Could you guide me how to reproduce the problem and test this contribution?

[voyageth]

@Leemoonsoo Oh, empty string also make weird graph too!
This PR doesn't consider that condition.

This PR can fix following condition.
%sh echo -e "%table key\tvalue\nnull\t1000\n1\t100\n2\t200\n99\t900"

• before apply PR
  
• after apply PR