Fix a segfault in fromModifiedJulianDay()

It was crashing when the argument type was Nullable(T) where T was any integral type other than Int32.

src/Functions/fromModifiedJulianDay.cpp

@@ -163,7 +163,7 @@ namespace DB
 
         FunctionBaseImplPtr build(const ColumnsWithTypeAndName & arguments, const DataTypePtr & return_type) const override
         {
-            const DataTypePtr & from_type = arguments[0].type;
+            const DataTypePtr & from_type = removeNullable(arguments[0].type);
             DataTypes argument_types = { from_type };
             FunctionBaseImplPtr base;
             auto call = [&](const auto & types) -> bool
@@ -185,7 +185,7 @@ namespace DB
                 * here causes a SEGV. So we must somehow create a
                 * dummy implementation and return it.
                 */
-            if (WhichDataType(from_type).isNullable()) // Nullable(Nothing)
+            if (WhichDataType(from_type).isNothing()) // Nullable(Nothing)
                 return std::make_unique<FunctionBaseFromModifiedJulianDay<Name, DataTypeInt32, nullOnErrors>>(argument_types, return_type);
             else
                 // Should not happen.

tests/queries/0_stateless/01544_fromModifiedJulianDay.reference

@@ -3,6 +3,7 @@ Invocation with constant
 1858-11-17
 2020-11-01
 \N
+\N
 or null
 2020-11-01
 \N

tests/queries/0_stateless/01544_fromModifiedJulianDay.sql

@@ -5,6 +5,7 @@ SELECT fromModifiedJulianDay(-1);
 SELECT fromModifiedJulianDay(0);
 SELECT fromModifiedJulianDay(59154);
 SELECT fromModifiedJulianDay(NULL);
+SELECT fromModifiedJulianDay(CAST(NULL, 'Nullable(Int64)'));
 SELECT fromModifiedJulianDay(-678942); -- { serverError 490 }
 SELECT fromModifiedJulianDay(2973484); -- { serverError 490 }