Fix Painless script used by TSVB

It's at the moment unclear if this is a Painless regression (looks like),
but this seems to fix it based on testing.

I'm doing this change in the 6.5 branch, for a couple of reasons: I tested
it in 6.5 only, because master didn't seem to exhibit this issue. And it is
not the right fix, if Painless has a true regression.

Closes elastic/beats#8957.

src/core_plugins/metrics/server/lib/vis_data/__tests__/helpers/bucket_transform.js

@@ -437,7 +437,7 @@ describe('bucketTransform', () => {
           },
           gap_policy: 'skip',
           script: {
-            source: 'params.value > 0 ? params.value : 0',
+            source: 'if (params.value > 0) {return params.value} else return 0',
             lang: 'painless'
           }
         }

src/core_plugins/metrics/server/lib/vis_data/helpers/bucket_transform.js

@@ -246,7 +246,7 @@ export default {
           value: getBucketsPath(bucket.field, metrics),
         },
         script: {
-          source: 'params.value > 0 ? params.value : 0',
+          source: 'if (params.value > 0) {return params.value} else return 0',
           lang: 'painless',
         },
         gap_policy: 'skip', // seems sane