Compare with previous value only if PYSON value is not null

issue10900
review385281003
(grafted from 5fd8d729330a81505f9390cc28fbf05153a84419)

--HG--
branch : 6.0

src/view/form.js

@@ -5110,7 +5110,7 @@ function eval_pyson(value){
             var record = this.record;
             var field = this.field;
             var previous = field.get_client(record);
-            if (previous && Sao.common.compare(
+            if (value && previous && Sao.common.compare(
                 value, this.encoder.encode(this.decoder.decode(previous)))) {
                 value = previous;
             }