Bug #30677984: WL#13476: UNINITIALISED VALUE ERROR WITH VALGRIND IN SQL_TMP_TABLE.CC

Field::val_real() must return 0.0 for NULL fields, and aggregation through
temporary tables relies on this. However, a recent bugfix to the TempTable
storage engine caused it to not copy in NULL fields from memory to the record
buffer. This would cause uninitialized memory to be read, giving garbage
results if the first input value in a group was NULL (but subsequent values
were not).

Fix by re-enabling copying of contents of NULL fields with fixed length,
like floats and doubles; the bug fix that caused the regression was only
valid for fields whose length fields were stored in the table.

Change-Id: Ib07cbb43e6f61e69eb1e5b51bd19b7dc546c0a86