Fix topk and bottomk operations with int <= 0

[ssncferreira]

What this PR does / why we need it:

How the problem was identified
Range mapper query fails for topk/bottomk expressions with parameter 0.
When parsing an expression the current flow is:

1. ParseSampleExpr from query string: https://github.com/grafana/loki/blob/main/pkg/logql/rangemapper.go#L80

	origExpr, err := syntax.ParseSampleExpr(query)

this operation succeeds with no error
2) ParseSampleExpr from expr.String(): https://github.com/grafana/loki/blob/main/pkg/logql/rangemapper.go#L451

	e, err := syntax.ParseSampleExpr(expr.String())

expr.String() does not consider the topk parameter 0 because of this condition: https://github.com/grafana/loki/blob/main/pkg/logql/syntax/ast.go#L927-L931

	if e.Params != 0 {
		params = []string{fmt.Sprintf("%d", e.Params), e.Left.String()}
	} else {
		params = []string{e.Left.String()}
	}

incorrectly converting the expression to string topk(sum by(geoip_country_code)(count_over_time({app="foo"} | json | ( geoip_country_code!="" , __error__="" )[15m]))) which then panic at https://github.com/grafana/loki/blob/main/pkg/logql/syntax/ast.go#L869-L871 because param is nil

		if params == nil {
			panic(logqlmodel.NewParseError(fmt.Sprintf("parameter required for operation %s", operation), 0, 0))
		}

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

The changes in this PR:

1. panic if the param is smaller or equal to 0, returning an error immediately on step 1.
2. fix expr.String() to consider the first parameter of topk/bottomk even if 0

Checklist

• Documentation added
• Tests updated
• Is this an important fix or new feature? Add an entry in the CHANGELOG.md.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/upgrading/_index.md

[grafanabot]

./tools/diff_coverage.sh ../loki-main/test_results.txt test_results.txt ingester,distributor,querier,querier/queryrange,iter,storage,chunkenc,logql,loki

Change in test coverage per package. Green indicates 0 or positive change, red indicates that test coverage for a package fell.

+           ingester	0%
+        distributor	0%
+            querier	0%
+ querier/queryrange	0%
+               iter	0%
+            storage	0%
+           chunkenc	0%
+              logql	0%
+               loki	0%

[grafanabot]

./tools/diff_coverage.sh ../loki-main/test_results.txt test_results.txt ingester,distributor,querier,querier/queryrange,iter,storage,chunkenc,logql,loki

Change in test coverage per package. Green indicates 0 or positive change, red indicates that test coverage for a package fell.

+           ingester	0%
+        distributor	0%
+            querier	0%
+ querier/queryrange	0%
+               iter	0%
+            storage	0%
+           chunkenc	0%
+              logql	0%
+               loki	0%

[grafanabot]

./tools/diff_coverage.sh ../loki-main/test_results.txt test_results.txt ingester,distributor,querier,querier/queryrange,iter,storage,chunkenc,logql,loki

Change in test coverage per package. Green indicates 0 or positive change, red indicates that test coverage for a package fell.

+           ingester	0%
+        distributor	0%
+            querier	0%
+ querier/queryrange	0%
+               iter	0%
+            storage	0%
+           chunkenc	0%
+              logql	0%
+               loki	0%

[MichelHollands]

Very nice, thanks for fixing this.

[grafanabot]

./tools/diff_coverage.sh ../loki-main/test_results.txt test_results.txt ingester,distributor,querier,querier/queryrange,iter,storage,chunkenc,logql,loki

Change in test coverage per package. Green indicates 0 or positive change, red indicates that test coverage for a package fell.

+           ingester	0%
+        distributor	0%
+            querier	0%
+ querier/queryrange	0%
+               iter	0%
+            storage	0%
+           chunkenc	0%
+              logql	0%
+               loki	0%

[owen-d]

LGTM, nice explanation & fix