throw exception when creating scramble on empty table

[Beastjoe]

Related Issue #351 VerdictDB will throw exception if empty table is used to create scramble. I do this by checking totalBlockSize in ScrambleNode.createQuery(). If totalBlockSize==0, it is equivalent to say the table size is 0.

[codecov-io]

Codecov Report

Merging #353 into master will increase coverage by 0.15%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #353      +/-   ##
==========================================
+ Coverage   71.02%   71.16%   +0.15%     
==========================================
  Files         168      168              
  Lines       11492    11487       -5     
  Branches     1874     1874              
==========================================
+ Hits         8161     8174      +13     
+ Misses       2849     2841       -8     
+ Partials      482      472      -10

Impacted Files	Coverage Δ
.../org/verdictdb/core/scrambling/ScramblingNode.java	94.32% <100%> (+0.14%)	:arrow_up:
...rg/verdictdb/exception/VerdictDBDbmsException.java	50% <0%> (-50%)	:arrow_down:
...ava/org/verdictdb/connection/SparkQueryResult.java	55.36% <0%> (-3.96%)	:arrow_down:
.../core/scrambling/FastConvergeScramblingMethod.java	89.02% <0%> (-3.46%)	:arrow_down:
.../verdictdb/core/execplan/ExecutableNodeRunner.java	83.7% <0%> (-3.43%)	:arrow_down:
...main/java/org/verdictdb/sqlsyntax/MysqlSyntax.java	85.3% <0%> (-2.94%)	:arrow_down:
...core/querying/ola/SelectAsyncAggExecutionNode.java	93.25% <0%> (-2.7%)	:arrow_down:
...java/org/verdictdb/connection/JdbcQueryResult.java	96.25% <0%> (-0.13%)	:arrow_down:
.../java/org/verdictdb/commons/DataTypeConverter.java	98.88% <0%> (-0.01%)	:arrow_down:
...c/main/java/org/verdictdb/VerdictSingleResult.java	77.78% <0%> (ø)	:arrow_up:
... and 17 more

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[dongyoungy]

I first thought we had to use different method for determining an empty table as opposed checking the size of the table, but I guess if it is an empty table, SELECT COUNT(*) will run quickly anyway so it seems fine to do it this way.