ESQL: Small fixes for COPY_SIGN (#132459)

Adds some small fixes to `COPY_SIGN` and some docs and tests. The fixes
are mostly supporting the literal `NULL` in parameters. `COPY_SIGN`
supports `float` arguments, but the tests don't cover it because we
don't build `float` blocks. Otherwise, the tests are fairly standard.

Author: nik9000

docs/changelog/132459.yaml

@@ -0,0 +1,5 @@
+pr: 132459
+summary: Small fixes for COPY_SIGN
+area: ES|QL
+type: bug
+issues: []

docs/reference/query-languages/esql/_snippets/functions/description/copy_sign.md

@@ -5,6 +5,7 @@
 * [`ATAN2`](../../functions-operators/math-functions.md#esql-atan2)
 * [`CBRT`](../../functions-operators/math-functions.md#esql-cbrt)
 * [`CEIL`](../../functions-operators/math-functions.md#esql-ceil)
+* [`COPY_SIGN`](../../functions-operators/math-functions.md#esql-copy_sign)
 * [`COS`](../../functions-operators/math-functions.md#esql-cos)
 * [`COSH`](../../functions-operators/math-functions.md#esql-cosh)
 * [`E`](../../functions-operators/math-functions.md#esql-e)

docs/reference/query-languages/esql/_snippets/functions/layout/copy_sign.md

@@ -33,6 +33,9 @@ mapped_pages:
 :::{include} ../_snippets/functions/layout/ceil.md
 :::
 
+:::{include} ../_snippets/functions/layout/copy_sign.md
+:::
+
 :::{include} ../_snippets/functions/layout/cos.md
 :::