Fix Custom Function Code Completion

[xterao]

When invoking code completion with the cursor placed between a custom function’s name and its parameter element, no suggestions appear. The logic that retrieves the element before the cursor returns null, so update the element-retrieval method to handle this case correctly.

doma-tools-for-intellij/src/main/kotlin/org/domaframework/doma/intellij/common/sql/directive/StaticDirectiveHandler.kt

Line 56 in 1432342

if (element.prevSibling?.elementType == SqlTypes.AT_SIGN) {