TMSS-761: enforce minimum of one SAP per observation

LTA/LTACommon/LTA-SIP.xsd

 <?xml version="1.0" encoding="UTF-8"?>
-<xs:schema targetNamespace="http://www.astron.nl/SIP-Lofar" version="2.7.2" xmlns="http://www.astron.nl/SIP-Lofar" xmlns:xs="http://www.w3.org/2001/XMLSchema">
+<xs:schema targetNamespace="http://www.astron.nl/SIP-Lofar" version="2.7.3" xmlns="http://www.astron.nl/SIP-Lofar" xmlns:xs="http://www.w3.org/2001/XMLSchema">
 	<xs:annotation>
 		<xs:documentation>
 		XML Schema for data model Submission Information Package LOFAR Long Term Archive
@@ -364,7 +364,7 @@
 					<xs:element name="numberOfStations" type="xs:unsignedByte"/>
 					<xs:element name="stations" type="Stations"/>
 					<xs:element name="numberOfSubArrayPointings" type="xs:unsignedShort"/>
-					<xs:element minOccurs="0" name="subArrayPointings" type="SubArrayPointings"/>
+					<xs:element minOccurs="1" name="subArrayPointings" type="SubArrayPointings"/>
 					<xs:element name="numberOftransientBufferBoardEvents" type="xs:unsignedShort"/>
 					<xs:element minOccurs="0" name="transientBufferBoardEvents" type="TransientBufferBoardEvents"/>
 					<xs:element name="numberOfCorrelatedDataProducts" type="xs:unsignedShort"/>
@@ -544,7 +544,7 @@
 	</xs:complexType>
 	<xs:complexType name="SubArrayPointings">
 		<xs:sequence>
-			<xs:element maxOccurs="unbounded" name="subArrayPointing" type="SubArrayPointing"/>
+			<xs:element minOccurs="1" maxOccurs="unbounded" name="subArrayPointing" type="SubArrayPointing"/>
 		</xs:sequence>
 	</xs:complexType>
 	<xs:annotation>

SAS/TMSS/backend/src/tmss/tmssapp/schemas/subtask_template-observation-1.json

@@ -215,7 +215,7 @@
                       "tabs"
                       ]
                     },
-                    "minItems": 0
+                    "minItems": 1
                   },
                   "stations": {
                     "description": "Stations to beam form. This can be a subset of the obervation stations.",

SAS/TMSS/backend/src/tmss/tmssapp/schemas/task_template-beamforming_observation-1.json

@@ -111,7 +111,7 @@
                 "description": "Which SAPs in the observation to beamform.",
                 "additionalItems": false,
                 "default": [],
-                "minItems": 0,
+                "minItems": 1,
                 "items": {
                   "type": "object",
                   "additionalProperties": false,