Allow clearing optional merchant field in merchant rules

The isInvalidMerchantValue check in TextBase.validate was rejecting
empty strings for the merchant rename field, even though the field is
optional. This prevented users from clearing a previously-saved merchant
value. Adding a truthiness guard so the validation only runs on
non-empty values.

Co-authored-by: Situ Chandra Shil <situchan@users.noreply.github.com>

Author: MelvinBot

src/components/Rule/TextBase.tsx

@@ -63,7 +63,7 @@ function TextBase<TFormID extends OnyxFormKey>({
             if (!isValid) {
                 (errors as Record<string, string>)[fieldID] = translate('common.error.characterLimitExceedCounter', byteLength, characterLimit);
             } else if (fieldID === CONST.EXPENSE_RULES.FIELDS.RENAME_MERCHANT || fieldID === CONST.MERCHANT_RULES.FIELDS.MERCHANT) {
-                if (isInvalidMerchantValue(trimmedValue)) {
+                if (trimmedValue && isInvalidMerchantValue(trimmedValue)) {
                     (errors as Record<string, string>)[fieldID] = translate('iou.error.invalidMerchant');
                 }
             }