@tunnus et al, please don't post screen shots of code/expressions or logs. Screen shots often end up displaying at a reduced size that makes them very difficult to read. Just copy-paste the actual text and use a fenced code block.
To the question, as @therealdb said, judicious use of the coalesce operators will generally help here, particularly if you apply local knowledge of the desired outcome. There's another one that's often helpful when working with sources for numeric values that can be occasionally null:
The ?# operator will take any left side expression and convert it to a number or the value on the right side, so someexp ?# -1 would produce -1 if the value of someexp can't be converted to a number or is null or NaN. In other words, the general form somexp ?# altval is shorthand for do local t=float(someexp), isvalue(t) ? t : altval done
You can use this to produce, for example, an "off scale low" value that wouldn't trip a "greater than" operator on a trigger for a temporary missing value on an entity. This would let you write your expression more tightly as:
round( getEntity( "..." )?.attributes?.current_sensor?.value ?# 0 *
getEntity( "..." )?.attributes?.x_hubitat_extra_values?.voltagePhase1 ?# 0, 0 )
Note that the ?# operator has higher precedence than the math operators (like * and +) so you don't need parentheses around the subexpression in this case.
That said, if it's specifically zero you're after where there's a null, you may not need to do anything special, because the expression language follows the JavaScript semantics for null in arithmetic: null is treated as zero (e.g. null * 8 = 0 * 8 = 0 and null + 8 = 0 + 8 = 8). So this could be written even shorter as:
round( getEntity( "..." )?.attributes?.current_sensor?.value *
getEntity( "..." )?.attributes?.x_hubitat_extra_values?.voltagePhase1, 0 )
